diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..eea27a3
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,69 @@
+name: CI
+
+on:
+  pull_request:
+
+jobs:
+  phpunit:
+    name: PHPUnit (PHP ${{ matrix.php }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        php: ['8.1', '8.2', '8.3', '8.4', '8.5']
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          extensions: dom, libxml, simplexml, mbstring, gd
+          coverage: none
+
+      - name: Install dependencies
+        run: composer install --prefer-dist --no-progress
+
+      - name: Run PHPUnit
+        run: composer test
+
+  phpstan:
+    name: PHPStan
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.3'
+          extensions: dom, libxml, simplexml
+          coverage: none
+
+      - name: Install dependencies
+        run: composer install --prefer-dist --no-progress
+
+      - name: Run PHPStan
+        run: composer phpstan
+
+  cs-fixer:
+    name: PHP CS Fixer
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.3'
+          extensions: dom, libxml, simplexml
+          coverage: none
+
+      - name: Install dependencies
+        run: composer install --prefer-dist --no-progress
+
+      - name: Run PHP CS Fixer
+        run: composer cs-check
diff --git a/.gitignore b/.gitignore
index 57872d0..8c3adde 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,6 @@
 /vendor/
+composer.lock
+.phpstan.cache
+.php-cs-fixer.cache
+.phpunit.cache
+.phpunit.result.cache
diff --git a/.php-cs-fixer.dist.php b/.php-cs-fixer.dist.php
new file mode 100644
index 0000000..cb19d78
--- /dev/null
+++ b/.php-cs-fixer.dist.php
@@ -0,0 +1,32 @@
+<?php
+
+$finder = (new PhpCsFixer\Finder())
+    ->in(__DIR__)
+    ->exclude('var')
+;
+
+return (new PhpCsFixer\Config())
+    ->setHideProgress(true)
+    ->setRules([
+        '@Symfony' => true,
+        '@PER-CS' => true,
+        'no_unused_imports' => true,
+        'php_unit_method_casing' => ['case' => 'snake_case'],
+        'global_namespace_import' => ['import_classes' => true],
+        'yoda_style' => [
+            'equal' => false,
+            'identical' => false,
+            'less_and_greater' => false,
+        ],
+        'trailing_comma_in_multiline' => [
+            'elements' => [
+                'arrays',
+                'arguments',
+                'parameters',
+            ],
+        ],
+        'native_function_invocation' => false,
+    ])
+    ->setUsingCache(false)
+    ->setFinder($finder)
+    ;
diff --git a/composer.json b/composer.json
index df46417..987bd29 100644
--- a/composer.json
+++ b/composer.json
@@ -2,18 +2,37 @@
     "name": "stann/factur-x",
     "type": "library",
     "require": {
+        "php": "^8.1",
         "mpdf/mpdf": "^8.1",
-        "ext-libxml": "*"
+        "ext-libxml": "*",
+        "ext-simplexml": "*",
+        "ext-dom": "*"
+    },
+    "require-dev": {
+        "phpunit/phpunit": "^10.0",
+        "phpstan/phpstan": "^2.1",
+        "friendsofphp/php-cs-fixer": "^3.92"
     },
     "autoload": {
         "psr-4": {
             "Stann\\FacturX\\": "src/"
         }
     },
+    "autoload-dev": {
+        "psr-4": {
+            "Stann\\FacturX\\Tests\\": "tests/"
+        }
+    },
     "authors": [
         {
             "name": "Camille Baronnet",
             "email": "git@camillebaronnet.fr"
         }
-    ]
+    ],
+    "scripts": {
+        "test": "phpunit",
+        "phpstan": "phpstan analyse --memory-limit=512M",
+        "cs-fix": "php-cs-fixer fix",
+        "cs-check": "php-cs-fixer fix --dry-run --diff"
+    }
 }
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..bb79a50
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,6 @@
+parameters:
+    level: max
+    paths:
+        - src
+        - tests
+    tmpDir: .phpstan.cache
diff --git a/phpunit.xml b/phpunit.xml
new file mode 100644
index 0000000..1c14620
--- /dev/null
+++ b/phpunit.xml
@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+         cacheDirectory=".phpunit.cache"
+         executionOrder="depends,defects"
+         requireCoverageMetadata="false"
+         beStrictAboutCoverageMetadata="false"
+         beStrictAboutOutputDuringTests="true"
+         failOnRisky="true"
+         failOnWarning="true">
+    <testsuites>
+        <testsuite name="default">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+
+    <source>
+        <include>
+            <directory>src</directory>
+        </include>
+    </source>
+</phpunit>
diff --git a/src/AppendixParts/ExchangedDocument.php b/src/AppendixParts/ExchangedDocument.php
index b42eff1..87154f0 100644
--- a/src/AppendixParts/ExchangedDocument.php
+++ b/src/AppendixParts/ExchangedDocument.php
@@ -1,10 +1,11 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX\AppendixParts;
 
 use DateTimeImmutable;
-use SimpleXMLElement;
-use Stann\FacturX\XmlSpecifications\Namespaces;
+use Stann\FacturX\XmlBuilder;
 
 class ExchangedDocument
 {
@@ -12,17 +13,13 @@ public function __construct(
         private readonly string $invoiceId,
         private readonly TypeCode $typeCode,
         private readonly DateTimeImmutable $issuedAt,
-    )
-    {
-    }
+    ) {}
 
-    public function toXml(SimpleXMLElement $element): void
+    public function toXml(XmlBuilder $xml): void
     {
-        $element->addChild('ram:ID', $this->invoiceId, Namespaces::NS_RAM->value);
-        $element->addChild('ram:TypeCode', $this->typeCode->value, Namespaces::NS_RAM->value);
-        $element->addChild('ram:IssueDateTime', null, Namespaces::NS_RAM->value)
-            ?->addChild('udt:DateTimeString', $this->issuedAt->format('Ymd'), Namespaces::NS_UDT->value)
-            ?->addAttribute('format', '102')
-        ;
+        $xml->ram('ID', $this->invoiceId);
+        $xml->ram('TypeCode', (string) $this->typeCode->value);
+        $xml->ram('IssueDateTime')
+            ->udt('DateTimeString', $this->issuedAt->format('Ymd'))->attr('format', '102');
     }
 }
diff --git a/src/AppendixParts/MonetarySummation.php b/src/AppendixParts/MonetarySummation.php
index 93227c9..c4d7c29 100644
--- a/src/AppendixParts/MonetarySummation.php
+++ b/src/AppendixParts/MonetarySummation.php
@@ -1,9 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX\AppendixParts;
 
-use SimpleXMLElement;
-use Stann\FacturX\XmlSpecifications\Namespaces;
+use Stann\FacturX\XmlBuilder;
 
 class MonetarySummation
 {
@@ -12,17 +13,15 @@ public function __construct(
         private readonly float $totalAmountExcludingTaxes,
         private readonly float $totalAmountIncludingTaxes,
         private readonly float $duePayableAmount,
-    )
-    {
-    }
+    ) {}
 
-    public function toXml(SimpleXMLElement $xml): void
+    public function toXml(XmlBuilder $xml): void
     {
-        $xml->addChild('ram:InvoiceCurrencyCode', $this->currency, Namespaces::NS_RAM->value);
-        $summation = $xml->addChild('ram:SpecifiedTradeSettlementHeaderMonetarySummation', null, Namespaces::NS_RAM->value);
-        $summation?->addChild('ram:TaxBasisTotalAmount', $this->totalAmountExcludingTaxes, Namespaces::NS_RAM->value);
-        $summation?->addChild('ram:TaxTotalAmount', $this->totalAmountIncludingTaxes - $this->totalAmountExcludingTaxes, Namespaces::NS_RAM->value);
-        $summation?->addChild('ram:GrandTotalAmount', $this->totalAmountIncludingTaxes, Namespaces::NS_RAM->value);
-        $summation?->addChild('ram:DuePayableAmount', $this->duePayableAmount, Namespaces::NS_RAM->value);
+        $xml->ram('InvoiceCurrencyCode', $this->currency);
+        $summation = $xml->ram('SpecifiedTradeSettlementHeaderMonetarySummation');
+        $summation->ram('TaxBasisTotalAmount', (string) $this->totalAmountExcludingTaxes);
+        $summation->ram('TaxTotalAmount', (string) ($this->totalAmountIncludingTaxes - $this->totalAmountExcludingTaxes));
+        $summation->ram('GrandTotalAmount', (string) $this->totalAmountIncludingTaxes);
+        $summation->ram('DuePayableAmount', (string) $this->duePayableAmount);
     }
 }
diff --git a/src/AppendixParts/TradeParty.php b/src/AppendixParts/TradeParty.php
index 209361f..5d5e9f7 100644
--- a/src/AppendixParts/TradeParty.php
+++ b/src/AppendixParts/TradeParty.php
@@ -1,43 +1,42 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX\AppendixParts;
 
-use SimpleXMLElement;
 use Stann\FacturX\ValueObjects\CountryCode;
 use Stann\FacturX\ValueObjects\VATNumber;
-use Stann\FacturX\XmlSpecifications\Namespaces;
+use Stann\FacturX\XmlBuilder;
 
 class TradeParty
 {
     public function __construct(
-        private readonly string       $legalName,
+        private readonly string $legalName,
         /**
          * For exemple in France, that's corresponding to the SIRET number.
          */
-        private readonly string       $registrationNumber,
+        private readonly string $registrationNumber,
         private readonly ?CountryCode $countryCode,
-        private readonly ?VATNumber   $vatNumber,
-    )
-    {
-    }
+        private readonly ?VATNumber $vatNumber,
+    ) {}
 
-    public function toXml(SimpleXMLElement $xml): void
+    public function toXml(XmlBuilder $xml): void
     {
-        $xml->addChild('ram:Name', $this->legalName, Namespaces::NS_RAM->value);
+        $xml->ram('Name', $this->legalName);
 
-        if(!empty($this->registrationNumber)) {
-            $xml->addChild('ram:SpecifiedLegalOrganization', null, Namespaces::NS_RAM->value)
-                ?->addChild('ram:ID', $this->registrationNumber, Namespaces::NS_RAM->value)?->addAttribute('schemeID', '0002');
+        if (!empty($this->registrationNumber)) {
+            $xml->ram('SpecifiedLegalOrganization')
+                ->ram('ID', $this->registrationNumber)->attr('schemeID', '0002');
         }
 
-        if(!empty($this->countryCode)) {
-            $xml->addChild('ram:PostalTradeAddress', null, Namespaces::NS_RAM->value)
-                ?->addChild('ram:CountryID', $this->countryCode->value, Namespaces::NS_RAM->value);
+        if (!empty($this->countryCode)) {
+            $xml->ram('PostalTradeAddress')
+                ->ram('CountryID', $this->countryCode->value);
         }
 
-        if(!empty($this->vatNumber)) {
-            $xml->addChild('ram:SpecifiedTaxRegistration', null, Namespaces::NS_RAM->value)
-                ?->addChild('ram:ID', $this->vatNumber->value, Namespaces::NS_RAM->value)?->addAttribute('schemeID', 'VA');
+        if (!empty($this->vatNumber)) {
+            $xml->ram('SpecifiedTaxRegistration')
+                ->ram('ID', $this->vatNumber->value)->attr('schemeID', 'VA');
         }
     }
 }
diff --git a/src/AppendixParts/TypeCode.php b/src/AppendixParts/TypeCode.php
index 3c97ce6..8860ffb 100644
--- a/src/AppendixParts/TypeCode.php
+++ b/src/AppendixParts/TypeCode.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX\AppendixParts;
 
 enum TypeCode: int
diff --git a/src/CrossIndustryInvoice.php b/src/CrossIndustryInvoice.php
index 7b8d064..57f87a0 100644
--- a/src/CrossIndustryInvoice.php
+++ b/src/CrossIndustryInvoice.php
@@ -1,7 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX;
 
+use DOMDocument;
 use SimpleXMLElement;
 use Stann\FacturX\AppendixParts\ExchangedDocument;
 use Stann\FacturX\AppendixParts\MonetarySummation;
@@ -20,7 +23,7 @@ public function __construct(
          * For Chorus Pro it is the "Service Exécutant". It is mandatory for some buyers.
          * It must belong to the Chorus Pro repository.
          *
-         * @link https://communaute.chorus-pro.gouv.fr/annuaire-cpro/
+         * @see https://communaute.chorus-pro.gouv.fr/annuaire-cpro/
          */
         private readonly string $buyerReference = '',
         /**
@@ -33,20 +36,18 @@ public function __construct(
          * For Chorus Pro Chorus Pro this is the "numéro de Marché".
          */
         private readonly string $buyerContractReference = '',
-    )
-    {
-    }
+    ) {}
 
     public function toXml(): string
     {
         $xml = new SimpleXMLElement(trim('
             <?xml version="1.0" encoding="UTF-8"?>
             <rsm:CrossIndustryInvoice 
-                    xmlns:qdt="'. Namespaces::NS_QDT->value .'" 
-                    xmlns:ram="'. Namespaces::NS_RAM->value .'" 
-                    xmlns:rsm="'. Namespaces::NS_RSM->value .'" 
-                    xmlns:udt="'. Namespaces::NS_UDT->value .'" 
-                    xmlns:xsi="'. Namespaces::NS_XSI->value .'">
+                    xmlns:qdt="' . Namespaces::NS_QDT->value . '" 
+                    xmlns:ram="' . Namespaces::NS_RAM->value . '" 
+                    xmlns:rsm="' . Namespaces::NS_RSM->value . '" 
+                    xmlns:udt="' . Namespaces::NS_UDT->value . '" 
+                    xmlns:xsi="' . Namespaces::NS_XSI->value . '">
                 <rsm:ExchangedDocumentContext>
                     <ram:GuidelineSpecifiedDocumentContextParameter>
                     <ram:ID>urn:factur-x.eu:1p0:minimum</ram:ID>
@@ -55,36 +56,35 @@ public function toXml(): string
             </rsm:CrossIndustryInvoice>
         '));
 
-        $this->document->toXml($xml->addChild('rsm:ExchangedDocument'));
+        $b = XmlBuilder::wrap($xml);
 
-        $supplyChainTradeTransaction = $xml->addChild('rsm:SupplyChainTradeTransaction', null, Namespaces::NS_RSM->value);
+        $this->document->toXml($b->rsm('ExchangedDocument'));
 
-        $applicableHeaderTradeAgreement = $supplyChainTradeTransaction?->addChild('ram:ApplicableHeaderTradeAgreement', null, Namespaces::NS_RAM->value);
+        $supplyChainTradeTransaction = $b->rsm('SupplyChainTradeTransaction');
+        $applicableHeaderTradeAgreement = $supplyChainTradeTransaction->ram('ApplicableHeaderTradeAgreement');
 
-        if(!empty($this->buyerReference)) {
-            $applicableHeaderTradeAgreement?->addChild('ram:BuyerReference', $this->buyerReference, Namespaces::NS_RAM->value);
+        if (!empty($this->buyerReference)) {
+            $applicableHeaderTradeAgreement->ram('BuyerReference', $this->buyerReference);
         }
 
-        $this->sellerTradeParty->toXml($applicableHeaderTradeAgreement?->addChild('ram:SellerTradeParty', null, Namespaces::NS_RAM->value));
-        $this->buyerTradeParty->toXml($applicableHeaderTradeAgreement?->addChild('ram:BuyerTradeParty', null, Namespaces::NS_RAM->value));
+        $this->sellerTradeParty->toXml($applicableHeaderTradeAgreement->ram('SellerTradeParty'));
+        $this->buyerTradeParty->toXml($applicableHeaderTradeAgreement->ram('BuyerTradeParty'));
 
-        if(!empty($this->buyerOrderReference)) {
-            $applicableHeaderTradeAgreement
-                ?->addChild('ram:BuyerOrderReferencedDocument', null, Namespaces::NS_RAM->value)
-                ?->addChild('ram:IssuerAssignedID', $this->buyerOrderReference, Namespaces::NS_RAM->value);
+        if (!empty($this->buyerOrderReference)) {
+            $applicableHeaderTradeAgreement->ram('BuyerOrderReferencedDocument')
+                ->ram('IssuerAssignedID', $this->buyerOrderReference);
         }
 
-        if(!empty($this->buyerContractReference)) {
-            $applicableHeaderTradeAgreement
-                ?->addChild('ram:ContractReferencedDocument', null, Namespaces::NS_RAM->value)
-                ?->addChild('ram:IssuerAssignedID', $this->buyerContractReference, Namespaces::NS_RAM->value);
+        if (!empty($this->buyerContractReference)) {
+            $applicableHeaderTradeAgreement->ram('ContractReferencedDocument')
+                ->ram('IssuerAssignedID', $this->buyerContractReference);
         }
 
-        $supplyChainTradeTransaction?->addChild('ram:ApplicableHeaderTradeDelivery', null, Namespaces::NS_RAM->value);
+        $supplyChainTradeTransaction->ram('ApplicableHeaderTradeDelivery');
 
-        $this->monetarySummation->toXml($supplyChainTradeTransaction?->addChild('ram:ApplicableHeaderTradeSettlement', null, Namespaces::NS_RAM->value));
+        $this->monetarySummation->toXml($supplyChainTradeTransaction->ram('ApplicableHeaderTradeSettlement'));
 
-        return $this->prettify($xml->asXML());
+        return $this->prettify((string) $xml->asXML());
     }
 
     public function __toString(): string
@@ -94,10 +94,11 @@ public function __toString(): string
 
     private function prettify(string $xml): string
     {
-        $doc = new \DOMDocument('1.0');
+        $doc = new DOMDocument('1.0');
         $doc->preserveWhiteSpace = false;
         $doc->formatOutput = true;
         $doc->loadXML($xml);
-        return $doc->saveXML();
+
+        return (string) $doc->saveXML();
     }
 }
diff --git a/src/Exception/InvalidXMLSchema.php b/src/Exception/InvalidXMLSchema.php
index 8c479d9..5c853a9 100644
--- a/src/Exception/InvalidXMLSchema.php
+++ b/src/Exception/InvalidXMLSchema.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX\Exception;
 
 use Exception;
@@ -12,6 +14,6 @@ class InvalidXMLSchema extends Exception
      */
     public function __construct(public readonly array $errors)
     {
-        parent::__construct("Invalid XML Schema");
+        parent::__construct('Invalid XML Schema');
     }
-}
\ No newline at end of file
+}
diff --git a/src/FacturX.php b/src/FacturX.php
index f84b9d2..c667b49 100644
--- a/src/FacturX.php
+++ b/src/FacturX.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX;
 
 use DOMDocument;
@@ -14,7 +16,6 @@ class FacturX
 {
     /**
      * @param resource|string $stream
-     * @return string
      */
     public function createFile($stream, string $appendix): string
     {
@@ -22,12 +23,12 @@ public function createFile($stream, string $appendix): string
             throw new RuntimeException('$stream must be a resource or a string');
         }
 
-        $mpdf = new Mpdf(['PDFA' => true, 'PDFAauto' => true,]);
+        $mpdf = new Mpdf(['PDFA' => true, 'PDFAauto' => true]);
         $pageCount = $mpdf->setSourceFile(
-            is_string($stream) ? StreamReader::createByString($stream) : $stream
+            is_string($stream) ? StreamReader::createByString($stream) : $stream,
         );
 
-        for ($i = 1; $i <= $pageCount; $i++) {
+        for ($i = 1; $i <= $pageCount; ++$i) {
             $mpdf->AddPage();
             $mpdf->UseTemplate($mpdf->ImportPage($i));
         }
@@ -39,12 +40,15 @@ public function createFile($stream, string $appendix): string
             'mime' => 'text/xml',
             'description' => 'Factur-X Invoice',
             'AFRelationship' => 'Alternative',
-            'content' => $appendix
+            'content' => $appendix,
         ]]);
 
         $mpdf->SetAdditionalXmpRdf(file_get_contents(__DIR__ . '/XmlSpecifications/Factur-X_extension_schema.xmp'));
 
-        return $mpdf->Output('', Destination::STRING_RETURN);
+        /** @var string $output */
+        $output = $mpdf->Output('', Destination::STRING_RETURN);
+
+        return $output;
     }
 
     /**
diff --git a/src/Schema.php b/src/Schema.php
index 8459340..476dd0b 100644
--- a/src/Schema.php
+++ b/src/Schema.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX;
 
 enum Schema: string
diff --git a/src/ValueObjects/CountryCode.php b/src/ValueObjects/CountryCode.php
index f5e523a..bbb6870 100644
--- a/src/ValueObjects/CountryCode.php
+++ b/src/ValueObjects/CountryCode.php
@@ -1,256 +1,258 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX\ValueObjects;
 
 enum CountryCode: string
 {
-    case AFGHANISTAN = "AF";
-    case ALAND_ISLANDS = "AX";
-    case ALBANIA = "AL";
-    case ALGERIA = "DZ";
-    case AMERICAN_SAMOA = "AS";
-    case ANDORRA = "AD";
-    case ANGOLA = "AO";
-    case ANGUILLA = "AI";
-    case ANTARCTICA = "AQ";
-    case ANTIGUA_AND_BARBUDA = "AG";
-    case ARGENTINA = "AR";
-    case ARMENIA = "AM";
-    case ARUBA = "AW";
-    case AUSTRALIA = "AU";
-    case AUSTRIA = "AT";
-    case AZERBAIJAN = "AZ";
-    case BAHAMAS = "BS";
-    case BAHRAIN = "BH";
-    case BANGLADESH = "BD";
-    case BARBADOS = "BB";
-    case BELARUS = "BY";
-    case BELGIUM = "BE";
-    case BELIZE = "BZ";
-    case BENIN = "BJ";
-    case BERMUDA = "BM";
-    case BHUTAN = "BT";
-    case BOLIVIA = "BO";
-    case BONAIRE = "BQ";
-    case BOSNIA_AND_HERZEGOVINA = "BA";
-    case BOTSWANA = "BW";
-    case BOUVET_ISLAND = "BV";
-    case BRAZIL = "BR";
-    case BRITISH_INDIAN_OCEAN_TERRITORY = "IO";
-    case BRUNEI_DARUSSALAM = "BN";
-    case BULGARIA = "BG";
-    case BURKINA_FASO = "BF";
-    case BURUNDI = "BI";
-    case CABO_VERDE = "CV";
-    case CAMBODIA = "KH";
-    case CAMEROON = "CM";
-    case CANADA = "CA";
-    case CAYMAN_ISLANDS = "KY";
-    case CENTRAL_AFRICAN_REPUBLIC = "CF";
-    case CHAD = "TD";
-    case CHILE = "CL";
-    case CHINA = "CN";
-    case CHRISTMAS_ISLAND = "CX";
-    case COCOS_ISLANDS = "CC";
-    case COLOMBIA = "CO";
-    case COMOROS = "KM";
-    case CONGO = "CG";
-    case CONGO_RD = "CD";
-    case COOK_ISLANDS = "CK";
-    case COSTA_RICA = "CR";
-    case COTE_D_IVOIRE = "CI";
-    case CROATIA = "HR";
-    case CUBA = "CU";
-    case CURACAO = "CW";
-    case CYPRUS = "CY";
-    case CZECHIA = "CZ";
-    case DENMARK = "DK";
-    case DJIBOUTI = "DJ";
-    case DOMINICA = "DM";
-    case DOMINICAN_REPUBLIC = "DO";
-    case ECUADOR = "EC";
-    case EGYPT = "EG";
-    case EL_SALVADOR = "SV";
-    case EQUATORIAL_GUINEA = "GQ";
-    case ERITREA = "ER";
-    case ESTONIA = "EE";
-    case ESWATINI = "SZ";
-    case ETHIOPIA = "ET";
-    case FALKLAND_ISLANDS = "FK";
-    case FAROE_ISLANDS = "FO";
-    case FIJI = "FJ";
-    case FINLAND = "FI";
-    case FRANCE = "FR";
-    case FRENCH_GUIANA = "GF";
-    case FRENCH_POLYNESIA = "PF";
-    case FRENCH_SOUTHERN_TERRITORIES = "TF";
-    case GABON = "GA";
-    case GAMBIA = "GM";
-    case GEORGIA = "GE";
-    case GERMANY = "DE";
-    case GHANA = "GH";
-    case GIBRALTAR = "GI";
-    case GREECE = "GR";
-    case GREENLAND = "GL";
-    case GRENADA = "GD";
-    case GUADELOUPE = "GP";
-    case GUAM = "GU";
-    case GUATEMALA = "GT";
-    case GUERNSEY = "GG";
-    case GUINEA = "GN";
-    case GUINEA_BISSAU = "GW";
-    case GUYANA = "GY";
-    case HAITI = "HT";
-    case HEARD_ISLAND_AND_MCDONALD_ISLANDS = "HM";
-    case HOLY_SEE = "VA";
-    case HONDURAS = "HN";
-    case HONG_KONG = "HK";
-    case HUNGARY = "HU";
-    case ICELAND = "IS";
-    case INDIA = "IN";
-    case INDONESIA = "ID";
-    case IRAN = "IR";
-    case IRAQ = "IQ";
-    case IRELAND = "IE";
-    case ISLE_OF_MAN = "IM";
-    case ISRAEL = "IL";
-    case ITALY = "IT";
-    case JAMAICA = "JM";
-    case JAPAN = "JP";
-    case JERSEY = "JE";
-    case JORDAN = "JO";
-    case KAZAKHSTAN = "KZ";
-    case KENYA = "KE";
-    case KIRIBATI = "KI";
-    case KOREA_RD = "KP";
-    case KOREA_R = "KR";
-    case KUWAIT = "KW";
-    case KYRGYZSTAN = "KG";
-    case LAO = "LA";
-    case LATVIA = "LV";
-    case LEBANON = "LB";
-    case LESOTHO = "LS";
-    case LIBERIA = "LR";
-    case LIBYA = "LY";
-    case LIECHTENSTEIN = "LI";
-    case LITHUANIA = "LT";
-    case LUXEMBOURG = "LU";
-    case MACAO = "MO";
-    case MADAGASCAR = "MG";
-    case MALAWI = "MW";
-    case MALAYSIA = "MY";
-    case MALDIVES = "MV";
-    case MALI = "ML";
-    case MALTA = "MT";
-    case MARSHALL_ISLANDS = "MH";
-    case MARTINIQUE = "MQ";
-    case MAURITANIA = "MR";
-    case MAURITIUS = "MU";
-    case MAYOTTE = "YT";
-    case MEXICO = "MX";
-    case MICRONESIA = "FM";
-    case MOLDOVA = "MD";
-    case MONACO = "MC";
-    case MONGOLIA = "MN";
-    case MONTENEGRO = "ME";
-    case MONTSERRAT = "MS";
-    case MOROCCO = "MA";
-    case MOZAMBIQUE = "MZ";
-    case MYANMAR = "MM";
-    case NAMIBIA = "NA";
-    case NAURU = "NR";
-    case NEPAL = "NP";
-    case NETHERLANDS = "NL";
-    case NEW_CALEDONIA = "NC";
-    case NEW_ZEALAND = "NZ";
-    case NICARAGUA = "NI";
-    case NIGER = "NE";
-    case NIGERIA = "NG";
-    case NIUE = "NU";
-    case NORFOLK_ISLAND = "NF";
-    case NORTH_MACEDONIA = "MK";
-    case NORTHERN_MARIANA_ISLANDS = "MP";
-    case NORWAY = "NO";
-    case OMAN = "OM";
-    case PAKISTAN = "PK";
-    case PALAU = "PW";
-    case PALESTINE = "PS";
-    case PANAMA = "PA";
-    case PAPUA_NEW_GUINEA = "PG";
-    case PARAGUAY = "PY";
-    case PERU = "PE";
-    case PHILIPPINES = "PH";
-    case PITCAIRN = "PN";
-    case POLAND = "PL";
-    case PORTUGAL = "PT";
-    case PUERTO_RICO = "PR";
-    case QATAR = "QA";
-    case REUNION = "RE";
-    case ROMANIA = "RO";
-    case RUSSIAN_FEDERATION = "RU";
-    case RWANDA = "RW";
-    case SAINT_BARTHELEMY = "BL";
-    case SAINT_HELENA = "SH";
-    case SAINT_KITTS_AND_NEVIS = "KN";
-    case SAINT_LUCIA = "LC";
-    case SAINT_MARTIN = "MF";
-    case SAINT_PIERRE_AND_MIQUELON = "PM";
-    case SAINT_VINCENT_AND_THE_GRENADINES = "VC";
-    case SAMOA = "WS";
-    case SAN_MARINO = "SM";
-    case SAO_TOME_AND_PRINCIPE = "ST";
-    case SAUDI_ARABIA = "SA";
-    case SENEGAL = "SN";
-    case SERBIA = "RS";
-    case SEYCHELLES = "SC";
-    case SIERRA_LEONE = "SL";
-    case SINGAPORE = "SG";
-    case SINT_MAARTEN = "SX";
-    case SLOVAKIA = "SK";
-    case SLOVENIA = "SI";
-    case SOLOMON_ISLANDS = "SB";
-    case SOMALIA = "SO";
-    case SOUTH_AFRICA = "ZA";
-    case SOUTH_GEORGIA_AND_THE_SOUTH_SANDWICH_ISLANDS = "GS";
-    case SOUTH_SUDAN = "SS";
-    case SPAIN = "ES";
-    case SRI_LANKA = "LK";
-    case SUDAN = "SD";
-    case SURINAME = "SR";
-    case SVALBARD_AND_JAN_MAYEN = "SJ";
-    case SWEDEN = "SE";
-    case SWITZERLAND = "CH";
-    case SYRIAN_ARAB_REPUBLIC = "SY";
-    case TAIWAN = "TW";
-    case TAJIKISTAN = "TJ";
-    case TANZANIA = "TZ";
-    case THAILAND = "TH";
-    case TIMOR_LESTE = "TL";
-    case TOGO = "TG";
-    case TOKELAU = "TK";
-    case TONGA = "TO";
-    case TRINIDAD_AND_TOBAGO = "TT";
-    case TUNISIA = "TN";
-    case TURKEY = "TR";
-    case TURKMENISTAN = "TM";
-    case TURKS_AND_CAICOS_ISLANDS = "TC";
-    case TUVALU = "TV";
-    case UGANDA = "UG";
-    case UKRAINE = "UA";
-    case UNITED_ARAB_EMIRATES = "AE";
-    case UNITED_KINGDOM_OF_GREAT_BRITAIN_AND_NORTHERN_IRELAND = "GB";
-    case UNITED_STATES_OF_AMERICA = "US";
-    case UNITED_STATES_MINOR_OUTLYING_ISLANDS = "UM";
-    case URUGUAY = "UY";
-    case UZBEKISTAN = "UZ";
-    case VANUATU = "VU";
-    case VENEZUELA = "VE";
-    case VIET_NAM = "VN";
-    case VIRGIN_ISLANDS_BRT = "VG";
-    case VIRGIN_ISLANDS_US = "VI";
-    case WALLIS_AND_FUTUNA = "WF";
-    case WESTERN_SAHARA = "EH";
-    case YEMEN = "YE";
-    case ZAMBIA = "ZM";
-    case ZIMBABWE = "ZW";
+    case AFGHANISTAN = 'AF';
+    case ALAND_ISLANDS = 'AX';
+    case ALBANIA = 'AL';
+    case ALGERIA = 'DZ';
+    case AMERICAN_SAMOA = 'AS';
+    case ANDORRA = 'AD';
+    case ANGOLA = 'AO';
+    case ANGUILLA = 'AI';
+    case ANTARCTICA = 'AQ';
+    case ANTIGUA_AND_BARBUDA = 'AG';
+    case ARGENTINA = 'AR';
+    case ARMENIA = 'AM';
+    case ARUBA = 'AW';
+    case AUSTRALIA = 'AU';
+    case AUSTRIA = 'AT';
+    case AZERBAIJAN = 'AZ';
+    case BAHAMAS = 'BS';
+    case BAHRAIN = 'BH';
+    case BANGLADESH = 'BD';
+    case BARBADOS = 'BB';
+    case BELARUS = 'BY';
+    case BELGIUM = 'BE';
+    case BELIZE = 'BZ';
+    case BENIN = 'BJ';
+    case BERMUDA = 'BM';
+    case BHUTAN = 'BT';
+    case BOLIVIA = 'BO';
+    case BONAIRE = 'BQ';
+    case BOSNIA_AND_HERZEGOVINA = 'BA';
+    case BOTSWANA = 'BW';
+    case BOUVET_ISLAND = 'BV';
+    case BRAZIL = 'BR';
+    case BRITISH_INDIAN_OCEAN_TERRITORY = 'IO';
+    case BRUNEI_DARUSSALAM = 'BN';
+    case BULGARIA = 'BG';
+    case BURKINA_FASO = 'BF';
+    case BURUNDI = 'BI';
+    case CABO_VERDE = 'CV';
+    case CAMBODIA = 'KH';
+    case CAMEROON = 'CM';
+    case CANADA = 'CA';
+    case CAYMAN_ISLANDS = 'KY';
+    case CENTRAL_AFRICAN_REPUBLIC = 'CF';
+    case CHAD = 'TD';
+    case CHILE = 'CL';
+    case CHINA = 'CN';
+    case CHRISTMAS_ISLAND = 'CX';
+    case COCOS_ISLANDS = 'CC';
+    case COLOMBIA = 'CO';
+    case COMOROS = 'KM';
+    case CONGO = 'CG';
+    case CONGO_RD = 'CD';
+    case COOK_ISLANDS = 'CK';
+    case COSTA_RICA = 'CR';
+    case COTE_D_IVOIRE = 'CI';
+    case CROATIA = 'HR';
+    case CUBA = 'CU';
+    case CURACAO = 'CW';
+    case CYPRUS = 'CY';
+    case CZECHIA = 'CZ';
+    case DENMARK = 'DK';
+    case DJIBOUTI = 'DJ';
+    case DOMINICA = 'DM';
+    case DOMINICAN_REPUBLIC = 'DO';
+    case ECUADOR = 'EC';
+    case EGYPT = 'EG';
+    case EL_SALVADOR = 'SV';
+    case EQUATORIAL_GUINEA = 'GQ';
+    case ERITREA = 'ER';
+    case ESTONIA = 'EE';
+    case ESWATINI = 'SZ';
+    case ETHIOPIA = 'ET';
+    case FALKLAND_ISLANDS = 'FK';
+    case FAROE_ISLANDS = 'FO';
+    case FIJI = 'FJ';
+    case FINLAND = 'FI';
+    case FRANCE = 'FR';
+    case FRENCH_GUIANA = 'GF';
+    case FRENCH_POLYNESIA = 'PF';
+    case FRENCH_SOUTHERN_TERRITORIES = 'TF';
+    case GABON = 'GA';
+    case GAMBIA = 'GM';
+    case GEORGIA = 'GE';
+    case GERMANY = 'DE';
+    case GHANA = 'GH';
+    case GIBRALTAR = 'GI';
+    case GREECE = 'GR';
+    case GREENLAND = 'GL';
+    case GRENADA = 'GD';
+    case GUADELOUPE = 'GP';
+    case GUAM = 'GU';
+    case GUATEMALA = 'GT';
+    case GUERNSEY = 'GG';
+    case GUINEA = 'GN';
+    case GUINEA_BISSAU = 'GW';
+    case GUYANA = 'GY';
+    case HAITI = 'HT';
+    case HEARD_ISLAND_AND_MCDONALD_ISLANDS = 'HM';
+    case HOLY_SEE = 'VA';
+    case HONDURAS = 'HN';
+    case HONG_KONG = 'HK';
+    case HUNGARY = 'HU';
+    case ICELAND = 'IS';
+    case INDIA = 'IN';
+    case INDONESIA = 'ID';
+    case IRAN = 'IR';
+    case IRAQ = 'IQ';
+    case IRELAND = 'IE';
+    case ISLE_OF_MAN = 'IM';
+    case ISRAEL = 'IL';
+    case ITALY = 'IT';
+    case JAMAICA = 'JM';
+    case JAPAN = 'JP';
+    case JERSEY = 'JE';
+    case JORDAN = 'JO';
+    case KAZAKHSTAN = 'KZ';
+    case KENYA = 'KE';
+    case KIRIBATI = 'KI';
+    case KOREA_RD = 'KP';
+    case KOREA_R = 'KR';
+    case KUWAIT = 'KW';
+    case KYRGYZSTAN = 'KG';
+    case LAO = 'LA';
+    case LATVIA = 'LV';
+    case LEBANON = 'LB';
+    case LESOTHO = 'LS';
+    case LIBERIA = 'LR';
+    case LIBYA = 'LY';
+    case LIECHTENSTEIN = 'LI';
+    case LITHUANIA = 'LT';
+    case LUXEMBOURG = 'LU';
+    case MACAO = 'MO';
+    case MADAGASCAR = 'MG';
+    case MALAWI = 'MW';
+    case MALAYSIA = 'MY';
+    case MALDIVES = 'MV';
+    case MALI = 'ML';
+    case MALTA = 'MT';
+    case MARSHALL_ISLANDS = 'MH';
+    case MARTINIQUE = 'MQ';
+    case MAURITANIA = 'MR';
+    case MAURITIUS = 'MU';
+    case MAYOTTE = 'YT';
+    case MEXICO = 'MX';
+    case MICRONESIA = 'FM';
+    case MOLDOVA = 'MD';
+    case MONACO = 'MC';
+    case MONGOLIA = 'MN';
+    case MONTENEGRO = 'ME';
+    case MONTSERRAT = 'MS';
+    case MOROCCO = 'MA';
+    case MOZAMBIQUE = 'MZ';
+    case MYANMAR = 'MM';
+    case NAMIBIA = 'NA';
+    case NAURU = 'NR';
+    case NEPAL = 'NP';
+    case NETHERLANDS = 'NL';
+    case NEW_CALEDONIA = 'NC';
+    case NEW_ZEALAND = 'NZ';
+    case NICARAGUA = 'NI';
+    case NIGER = 'NE';
+    case NIGERIA = 'NG';
+    case NIUE = 'NU';
+    case NORFOLK_ISLAND = 'NF';
+    case NORTH_MACEDONIA = 'MK';
+    case NORTHERN_MARIANA_ISLANDS = 'MP';
+    case NORWAY = 'NO';
+    case OMAN = 'OM';
+    case PAKISTAN = 'PK';
+    case PALAU = 'PW';
+    case PALESTINE = 'PS';
+    case PANAMA = 'PA';
+    case PAPUA_NEW_GUINEA = 'PG';
+    case PARAGUAY = 'PY';
+    case PERU = 'PE';
+    case PHILIPPINES = 'PH';
+    case PITCAIRN = 'PN';
+    case POLAND = 'PL';
+    case PORTUGAL = 'PT';
+    case PUERTO_RICO = 'PR';
+    case QATAR = 'QA';
+    case REUNION = 'RE';
+    case ROMANIA = 'RO';
+    case RUSSIAN_FEDERATION = 'RU';
+    case RWANDA = 'RW';
+    case SAINT_BARTHELEMY = 'BL';
+    case SAINT_HELENA = 'SH';
+    case SAINT_KITTS_AND_NEVIS = 'KN';
+    case SAINT_LUCIA = 'LC';
+    case SAINT_MARTIN = 'MF';
+    case SAINT_PIERRE_AND_MIQUELON = 'PM';
+    case SAINT_VINCENT_AND_THE_GRENADINES = 'VC';
+    case SAMOA = 'WS';
+    case SAN_MARINO = 'SM';
+    case SAO_TOME_AND_PRINCIPE = 'ST';
+    case SAUDI_ARABIA = 'SA';
+    case SENEGAL = 'SN';
+    case SERBIA = 'RS';
+    case SEYCHELLES = 'SC';
+    case SIERRA_LEONE = 'SL';
+    case SINGAPORE = 'SG';
+    case SINT_MAARTEN = 'SX';
+    case SLOVAKIA = 'SK';
+    case SLOVENIA = 'SI';
+    case SOLOMON_ISLANDS = 'SB';
+    case SOMALIA = 'SO';
+    case SOUTH_AFRICA = 'ZA';
+    case SOUTH_GEORGIA_AND_THE_SOUTH_SANDWICH_ISLANDS = 'GS';
+    case SOUTH_SUDAN = 'SS';
+    case SPAIN = 'ES';
+    case SRI_LANKA = 'LK';
+    case SUDAN = 'SD';
+    case SURINAME = 'SR';
+    case SVALBARD_AND_JAN_MAYEN = 'SJ';
+    case SWEDEN = 'SE';
+    case SWITZERLAND = 'CH';
+    case SYRIAN_ARAB_REPUBLIC = 'SY';
+    case TAIWAN = 'TW';
+    case TAJIKISTAN = 'TJ';
+    case TANZANIA = 'TZ';
+    case THAILAND = 'TH';
+    case TIMOR_LESTE = 'TL';
+    case TOGO = 'TG';
+    case TOKELAU = 'TK';
+    case TONGA = 'TO';
+    case TRINIDAD_AND_TOBAGO = 'TT';
+    case TUNISIA = 'TN';
+    case TURKEY = 'TR';
+    case TURKMENISTAN = 'TM';
+    case TURKS_AND_CAICOS_ISLANDS = 'TC';
+    case TUVALU = 'TV';
+    case UGANDA = 'UG';
+    case UKRAINE = 'UA';
+    case UNITED_ARAB_EMIRATES = 'AE';
+    case UNITED_KINGDOM_OF_GREAT_BRITAIN_AND_NORTHERN_IRELAND = 'GB';
+    case UNITED_STATES_OF_AMERICA = 'US';
+    case UNITED_STATES_MINOR_OUTLYING_ISLANDS = 'UM';
+    case URUGUAY = 'UY';
+    case UZBEKISTAN = 'UZ';
+    case VANUATU = 'VU';
+    case VENEZUELA = 'VE';
+    case VIET_NAM = 'VN';
+    case VIRGIN_ISLANDS_BRT = 'VG';
+    case VIRGIN_ISLANDS_US = 'VI';
+    case WALLIS_AND_FUTUNA = 'WF';
+    case WESTERN_SAHARA = 'EH';
+    case YEMEN = 'YE';
+    case ZAMBIA = 'ZM';
+    case ZIMBABWE = 'ZW';
 }
diff --git a/src/ValueObjects/VATNumber.php b/src/ValueObjects/VATNumber.php
index 3227ea7..bd09575 100644
--- a/src/ValueObjects/VATNumber.php
+++ b/src/ValueObjects/VATNumber.php
@@ -1,10 +1,10 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX\ValueObjects;
 
 class VATNumber
 {
-    public function __construct(public readonly string $value)
-    {
-    }
+    public function __construct(public readonly string $value) {}
 }
diff --git a/src/XmlBuilder.php b/src/XmlBuilder.php
new file mode 100644
index 0000000..f5157a2
--- /dev/null
+++ b/src/XmlBuilder.php
@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stann\FacturX;
+
+use SimpleXMLElement;
+use Stann\FacturX\XmlSpecifications\Namespaces;
+
+class XmlBuilder
+{
+    public function __construct(private readonly SimpleXMLElement $element) {}
+
+    public static function wrap(SimpleXMLElement $element): self
+    {
+        return new self($element);
+    }
+
+    public function ram(string $name, ?string $value = null): self
+    {
+        $child = $this->element->addChild("ram:$name", $value, Namespaces::NS_RAM->value);
+
+        return new self($child);
+    }
+
+    public function udt(string $name, ?string $value = null): self
+    {
+        $child = $this->element->addChild("udt:$name", $value, Namespaces::NS_UDT->value);
+
+        return new self($child);
+    }
+
+    public function rsm(string $name, ?string $value = null): self
+    {
+        $child = $this->element->addChild("rsm:$name", $value, Namespaces::NS_RSM->value);
+
+        return new self($child);
+    }
+
+    public function attr(string $name, string $value): self
+    {
+        $this->element->addAttribute($name, $value);
+
+        return $this;
+    }
+
+    public function unwrap(): SimpleXMLElement
+    {
+        return $this->element;
+    }
+}
diff --git a/src/XmlSpecifications/Namespaces.php b/src/XmlSpecifications/Namespaces.php
index ff3c85a..b9e35e2 100644
--- a/src/XmlSpecifications/Namespaces.php
+++ b/src/XmlSpecifications/Namespaces.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Stann\FacturX\XmlSpecifications;
 
 /**
diff --git a/tests/FacturXTest.php b/tests/FacturXTest.php
new file mode 100644
index 0000000..fa797e9
--- /dev/null
+++ b/tests/FacturXTest.php
@@ -0,0 +1,252 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stann\FacturX\Tests;
+
+use Mpdf\Mpdf;
+use Mpdf\Output\Destination;
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+use Stann\FacturX\Exception\InvalidXMLSchema;
+use Stann\FacturX\FacturX;
+use Stann\FacturX\Schema;
+use LibXMLError;
+
+class FacturXTest extends TestCase
+{
+    private FacturX $facturX;
+
+    protected function setUp(): void
+    {
+        $this->facturX = new FacturX();
+    }
+
+    // ========================================
+    // Tests for validate()
+    // ========================================
+
+    public function test_validate_throws_exception_for_invalid_xml(): void
+    {
+        $invalidXml = '<?xml version="1.0"?><invalid>content</invalid>';
+
+        $this->expectException(InvalidXMLSchema::class);
+        $this->facturX->validate($invalidXml, Schema::MINIMUM);
+    }
+
+    public function test_validate_does_not_throw_for_valid_xml(): void
+    {
+        $validXml = $this->getValidMinimumXml();
+
+        $this->facturX->validate($validXml, Schema::MINIMUM);
+        $this->expectNotToPerformAssertions();
+    }
+
+    public function test_validate_exception_contains_errors(): void
+    {
+        $invalidXml = '<?xml version="1.0"?><invalid>content</invalid>';
+
+        try {
+            $this->facturX->validate($invalidXml, Schema::MINIMUM);
+            $this->fail('Expected InvalidXMLSchema exception');
+        } catch (InvalidXMLSchema $e) {
+            $this->assertNotEmpty($e->errors);
+            $this->assertContainsOnlyInstancesOf(LibXMLError::class, $e->errors);
+        }
+    }
+
+    // ========================================
+    // Tests for isValid()
+    // ========================================
+
+    public function test_is_valid_returns_true_for_valid_xml(): void
+    {
+        $validXml = $this->getValidMinimumXml();
+
+        $this->assertTrue($this->facturX->isValid($validXml, Schema::MINIMUM));
+    }
+
+    public function test_is_valid_returns_false_for_invalid_xml(): void
+    {
+        $invalidXml = '<?xml version="1.0"?><invalid>content</invalid>';
+
+        $this->assertFalse($this->facturX->isValid($invalidXml, Schema::MINIMUM));
+    }
+
+    public function test_is_valid_returns_false_for_wrong_schema(): void
+    {
+        $validXml = $this->getValidMinimumXml();
+
+        // MINIMUM XML should not validate against BASIC_WL schema
+        $this->assertFalse($this->facturX->isValid($validXml, Schema::BASIC_WL));
+    }
+
+    // ========================================
+    // Tests for getValidationErrors()
+    // ========================================
+
+    public function test_get_validation_errors_returns_empty_array_for_valid_xml(): void
+    {
+        $validXml = $this->getValidMinimumXml();
+
+        $errors = $this->facturX->getValidationErrors($validXml, Schema::MINIMUM);
+
+        $this->assertEmpty($errors);
+    }
+
+    public function test_get_validation_errors_returns_errors_for_invalid_xml(): void
+    {
+        $invalidXml = '<?xml version="1.0"?><invalid>content</invalid>';
+
+        $errors = $this->facturX->getValidationErrors($invalidXml, Schema::MINIMUM);
+
+        $this->assertNotEmpty($errors);
+        $this->assertContainsOnlyInstancesOf(LibXMLError::class, $errors);
+    }
+
+    // ========================================
+    // Tests for createFile()
+    // ========================================
+
+    public function test_create_file_with_string_pdf(): void
+    {
+        $pdfContent = $this->generateValidPdf();
+        $xmlAppendix = $this->getValidMinimumXml();
+
+        $result = $this->facturX->createFile($pdfContent, $xmlAppendix);
+
+        $this->assertStringStartsWith('%PDF', $result);
+    }
+
+    public function test_create_file_with_resource(): void
+    {
+        $pdfContent = $this->generateValidPdf();
+        $stream = fopen('php://memory', 'r+');
+        $this->assertIsResource($stream);
+        fwrite($stream, $pdfContent);
+        rewind($stream);
+
+        $xmlAppendix = $this->getValidMinimumXml();
+
+        $result = $this->facturX->createFile($stream, $xmlAppendix);
+
+        fclose($stream);
+
+        $this->assertStringStartsWith('%PDF', $result);
+    }
+
+    public function test_create_file_throws_exception_for_invalid_stream_type(): void
+    {
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('$stream must be a resource or a string');
+
+        // @phpstan-ignore-next-line Testing invalid type
+        $this->facturX->createFile(12345, '<xml/>');
+    }
+
+    public function test_create_file_throws_exception_for_null(): void
+    {
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('$stream must be a resource or a string');
+
+        // @phpstan-ignore-next-line Testing invalid type
+        $this->facturX->createFile(null, '<xml/>');
+    }
+
+    public function test_create_file_throws_exception_for_array(): void
+    {
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('$stream must be a resource or a string');
+
+        // @phpstan-ignore-next-line Testing invalid type
+        $this->facturX->createFile(['invalid'], '<xml/>');
+    }
+
+    public function test_create_file_output_contains_xml_appendix(): void
+    {
+        $pdfContent = $this->generateValidPdf();
+        $xmlAppendix = $this->getValidMinimumXml();
+
+        $result = $this->facturX->createFile($pdfContent, $xmlAppendix);
+
+        // The PDF should contain the factur-x.xml filename reference
+        $this->assertStringContainsString('factur-x.xml', $result);
+    }
+
+    // ========================================
+    // Helper methods
+    // ========================================
+
+    private function getValidMinimumXml(): string
+    {
+        return <<<XML
+<?xml version="1.0" encoding="UTF-8"?>
+<rsm:CrossIndustryInvoice xmlns:rsm="urn:un:unece:uncefact:data:standard:CrossIndustryInvoice:100"
+    xmlns:qdt="urn:un:unece:uncefact:data:standard:QualifiedDataType:100"
+    xmlns:ram="urn:un:unece:uncefact:data:standard:ReusableAggregateBusinessInformationEntity:100"
+    xmlns:xs="http://www.w3.org/2001/XMLSchema"
+    xmlns:udt="urn:un:unece:uncefact:data:standard:UnqualifiedDataType:100">
+    <rsm:ExchangedDocumentContext>
+        <ram:GuidelineSpecifiedDocumentContextParameter>
+            <ram:ID>urn:factur-x.eu:1p0:minimum</ram:ID>
+        </ram:GuidelineSpecifiedDocumentContextParameter>
+    </rsm:ExchangedDocumentContext>
+    <rsm:ExchangedDocument>
+        <ram:ID>F0001</ram:ID>
+        <ram:TypeCode>380</ram:TypeCode>
+        <ram:IssueDateTime>
+            <udt:DateTimeString format="102">20240115</udt:DateTimeString>
+        </ram:IssueDateTime>
+    </rsm:ExchangedDocument>
+    <rsm:SupplyChainTradeTransaction>
+        <ram:ApplicableHeaderTradeAgreement>
+            <ram:SellerTradeParty>
+                <ram:Name>Seller Company</ram:Name>
+                <ram:SpecifiedLegalOrganization>
+                    <ram:ID>12345678901234</ram:ID>
+                </ram:SpecifiedLegalOrganization>
+                <ram:PostalTradeAddress>
+                    <ram:CountryID>FR</ram:CountryID>
+                </ram:PostalTradeAddress>
+                <ram:SpecifiedTaxRegistration>
+                    <ram:ID schemeID="VA">FR12345678901</ram:ID>
+                </ram:SpecifiedTaxRegistration>
+            </ram:SellerTradeParty>
+            <ram:BuyerTradeParty>
+                <ram:Name>Buyer Company</ram:Name>
+                <ram:SpecifiedLegalOrganization>
+                    <ram:ID>98765432109876</ram:ID>
+                </ram:SpecifiedLegalOrganization>
+                <ram:PostalTradeAddress>
+                    <ram:CountryID>FR</ram:CountryID>
+                </ram:PostalTradeAddress>
+                <ram:SpecifiedTaxRegistration>
+                    <ram:ID schemeID="VA">FR98765432109</ram:ID>
+                </ram:SpecifiedTaxRegistration>
+            </ram:BuyerTradeParty>
+        </ram:ApplicableHeaderTradeAgreement>
+        <ram:ApplicableHeaderTradeDelivery/>
+        <ram:ApplicableHeaderTradeSettlement>
+            <ram:InvoiceCurrencyCode>EUR</ram:InvoiceCurrencyCode>
+            <ram:SpecifiedTradeSettlementHeaderMonetarySummation>
+                <ram:TaxBasisTotalAmount>100.00</ram:TaxBasisTotalAmount>
+                <ram:GrandTotalAmount>120.00</ram:GrandTotalAmount>
+                <ram:DuePayableAmount>120.00</ram:DuePayableAmount>
+            </ram:SpecifiedTradeSettlementHeaderMonetarySummation>
+        </ram:ApplicableHeaderTradeSettlement>
+    </rsm:SupplyChainTradeTransaction>
+</rsm:CrossIndustryInvoice>
+XML;
+    }
+
+    private function generateValidPdf(): string
+    {
+        $mpdf = new Mpdf(['tempDir' => sys_get_temp_dir()]);
+        $mpdf->WriteHTML('<h1>Test Invoice</h1>');
+
+        /** @var string $output */
+        $output = $mpdf->Output('', Destination::STRING_RETURN);
+
+        return $output;
+    }
+}
diff --git a/tests/SchemaMinimumTest.php b/tests/SchemaMinimumTest.php
new file mode 100644
index 0000000..babac71
--- /dev/null
+++ b/tests/SchemaMinimumTest.php
@@ -0,0 +1,251 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Stann\FacturX\Tests;
+
+use DateTimeImmutable;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+use Stann\FacturX\AppendixParts\ExchangedDocument;
+use Stann\FacturX\AppendixParts\MonetarySummation;
+use Stann\FacturX\AppendixParts\TradeParty;
+use Stann\FacturX\AppendixParts\TypeCode;
+use Stann\FacturX\CrossIndustryInvoice;
+use Stann\FacturX\FacturX;
+use Stann\FacturX\Schema;
+use Stann\FacturX\ValueObjects\CountryCode;
+use Stann\FacturX\ValueObjects\VATNumber;
+
+class SchemaMinimumTest extends TestCase
+{
+    private FacturX $facturX;
+
+    protected function setUp(): void
+    {
+        $this->facturX = new FacturX();
+    }
+
+    #[DataProvider('validMinimumInvoicesProvider')]
+    public function test_valid_minimum_invoices(CrossIndustryInvoice $invoice, string $description): void
+    {
+        $xml = $invoice->toXml();
+        $errors = $this->facturX->getValidationErrors($xml, Schema::MINIMUM);
+
+        $this->assertEmpty(
+            $errors,
+            sprintf(
+                "Invoice '%s' should be valid against MINIMUM schema.\nErrors: %s",
+                $description,
+                implode("\n", array_map(fn($e) => $e->message, $errors)),
+            ),
+        );
+    }
+
+    #[DataProvider('invalidMinimumInvoicesProvider')]
+    public function test_invalid_minimum_invoices(CrossIndustryInvoice $invoice, string $description): void
+    {
+        $xml = $invoice->toXml();
+
+        $this->assertFalse(
+            $this->facturX->isValid($xml, Schema::MINIMUM),
+            sprintf("Invoice '%s' should NOT be valid against MINIMUM schema.", $description),
+        );
+    }
+
+    /**
+     * @return iterable<string, array{CrossIndustryInvoice, string}>
+     */
+    public static function validMinimumInvoicesProvider(): iterable
+    {
+        yield 'basic invoice' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('F0001', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Seller Company', '12345678901234', CountryCode::FRANCE, new VATNumber('FR12345678901')),
+                buyerTradeParty: new TradeParty('Buyer Company', '98765432109876', CountryCode::FRANCE, new VATNumber('FR98765432109')),
+                monetarySummation: new MonetarySummation('EUR', 100.00, 120.00, 120.00),
+            ),
+            'basic invoice',
+        ];
+
+        yield 'invoice with buyer reference (Chorus Pro)' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('F0002', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Stann', '34261828837536', CountryCode::FRANCE, new VATNumber('FR520000000')),
+                buyerTradeParty: new TradeParty('Mairie de Toulouse', '10881392400937', CountryCode::FRANCE, new VATNumber('FR980000000')),
+                monetarySummation: new MonetarySummation('EUR', 100.00, 120.00, 120.00),
+                buyerReference: '1004',
+            ),
+            'invoice with buyer reference (Chorus Pro)',
+        ];
+
+        yield 'invoice with buyer reference and order reference' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('F0003', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Stann', '34261828837536', CountryCode::FRANCE, new VATNumber('FR520000000')),
+                buyerTradeParty: new TradeParty('Mairie de Toulouse', '10881392400937', CountryCode::FRANCE, new VATNumber('FR980000000')),
+                monetarySummation: new MonetarySummation('EUR', 100.00, 120.00, 120.00),
+                buyerReference: '1004',
+                buyerOrderReference: 'BDC000001',
+            ),
+            'invoice with buyer reference and order reference',
+        ];
+
+        yield 'credit note' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('AV0001', TypeCode::CREDIT_NOTE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Seller', '12345678901234', CountryCode::FRANCE, new VATNumber('FR12345678901')),
+                buyerTradeParty: new TradeParty('Buyer', '98765432109876', CountryCode::FRANCE, new VATNumber('FR98765432109')),
+                monetarySummation: new MonetarySummation('EUR', -100.00, -120.00, -120.00),
+            ),
+            'credit note',
+        ];
+
+        yield 'invoice with German parties' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('DE-001', TypeCode::INVOICE, new DateTimeImmutable('2024-02-20')),
+                sellerTradeParty: new TradeParty('Deutsche Firma GmbH', 'HRB123456', CountryCode::GERMANY, new VATNumber('DE123456789')),
+                buyerTradeParty: new TradeParty('Käufer AG', 'HRB654321', CountryCode::GERMANY, new VATNumber('DE987654321')),
+                monetarySummation: new MonetarySummation('EUR', 500.00, 595.00, 595.00),
+            ),
+            'invoice with German parties',
+        ];
+
+        yield 'invoice with different currencies (USD)' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('USD-001', TypeCode::INVOICE, new DateTimeImmutable('2024-03-01')),
+                sellerTradeParty: new TradeParty('US Seller Inc', '123456789', CountryCode::UNITED_STATES_OF_AMERICA, new VATNumber('US123456789')),
+                buyerTradeParty: new TradeParty('US Buyer LLC', '987654321', CountryCode::UNITED_STATES_OF_AMERICA, new VATNumber('US987654321')),
+                monetarySummation: new MonetarySummation('USD', 1000.00, 1100.00, 1100.00),
+            ),
+            'invoice with different currencies (USD)',
+        ];
+
+        yield 'invoice with zero tax' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('NOTAX-001', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Seller', '12345678901234', CountryCode::FRANCE, new VATNumber('FR12345678901')),
+                buyerTradeParty: new TradeParty('Buyer', '98765432109876', CountryCode::FRANCE, new VATNumber('FR98765432109')),
+                monetarySummation: new MonetarySummation('EUR', 100.00, 100.00, 100.00),
+            ),
+            'invoice with zero tax',
+        ];
+
+        yield 'invoice with large amounts' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('BIG-001', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Big Corp', '12345678901234', CountryCode::FRANCE, new VATNumber('FR12345678901')),
+                buyerTradeParty: new TradeParty('Mega Corp', '98765432109876', CountryCode::FRANCE, new VATNumber('FR98765432109')),
+                monetarySummation: new MonetarySummation('EUR', 999999999.99, 1199999999.99, 1199999999.99),
+            ),
+            'invoice with large amounts',
+        ];
+
+        yield 'self-billing invoice' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('SB-001', TypeCode::SELF_BILLING_INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Seller', '12345678901234', CountryCode::FRANCE, new VATNumber('FR12345678901')),
+                buyerTradeParty: new TradeParty('Buyer', '98765432109876', CountryCode::FRANCE, new VATNumber('FR98765432109')),
+                monetarySummation: new MonetarySummation('EUR', 100.00, 120.00, 120.00),
+            ),
+            'self-billing invoice',
+        ];
+
+        yield 'deposit invoice' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('DEP-001', TypeCode::INVOICE_DEPOSIT, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Seller', '12345678901234', CountryCode::FRANCE, new VATNumber('FR12345678901')),
+                buyerTradeParty: new TradeParty('Buyer', '98765432109876', CountryCode::FRANCE, new VATNumber('FR98765432109')),
+                monetarySummation: new MonetarySummation('EUR', 500.00, 600.00, 600.00),
+            ),
+            'deposit invoice',
+        ];
+    }
+
+    /**
+     * @return iterable<string, array{CrossIndustryInvoice, string}>
+     */
+    public static function invalidMinimumInvoicesProvider(): iterable
+    {
+        yield 'invoice with ContractReferencedDocument (not allowed in MINIMUM)' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('F0001', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Seller', '12345678901234', CountryCode::FRANCE, new VATNumber('FR12345678901')),
+                buyerTradeParty: new TradeParty('Buyer', '98765432109876', CountryCode::FRANCE, new VATNumber('FR98765432109')),
+                monetarySummation: new MonetarySummation('EUR', 100.00, 120.00, 120.00),
+                buyerContractReference: 'CONTRACT-001',
+            ),
+            'invoice with ContractReferencedDocument (not allowed in MINIMUM)',
+        ];
+    }
+
+    /**
+     * @return iterable<string, array{CrossIndustryInvoice, string}>
+     */
+    public static function validMinimumOptionalFieldsProvider(): iterable
+    {
+        yield 'invoice without VAT number (optional in MINIMUM)' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('F0001', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Seller', '12345678901234', CountryCode::FRANCE, null),
+                buyerTradeParty: new TradeParty('Buyer', '98765432109876', CountryCode::FRANCE, null),
+                monetarySummation: new MonetarySummation('EUR', 100.00, 120.00, 120.00),
+            ),
+            'invoice without VAT number (optional in MINIMUM)',
+        ];
+
+        yield 'invoice without country code (optional in MINIMUM)' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('F0001', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Seller', '12345678901234', null, new VATNumber('FR12345678901')),
+                buyerTradeParty: new TradeParty('Buyer', '98765432109876', null, new VATNumber('FR98765432109')),
+                monetarySummation: new MonetarySummation('EUR', 100.00, 120.00, 120.00),
+            ),
+            'invoice without country code (optional in MINIMUM)',
+        ];
+
+        yield 'invoice with minimal data (only required fields)' => [
+            new CrossIndustryInvoice(
+                document: new ExchangedDocument('F0001', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+                sellerTradeParty: new TradeParty('Seller', '', null, null),
+                buyerTradeParty: new TradeParty('Buyer', '', null, null),
+                monetarySummation: new MonetarySummation('EUR', 100.00, 120.00, 120.00),
+            ),
+            'invoice with minimal data (only required fields)',
+        ];
+    }
+
+    #[DataProvider('validMinimumOptionalFieldsProvider')]
+    public function test_valid_minimum_with_optional_fields(CrossIndustryInvoice $invoice, string $description): void
+    {
+        $xml = $invoice->toXml();
+        $errors = $this->facturX->getValidationErrors($xml, Schema::MINIMUM);
+
+        $this->assertEmpty(
+            $errors,
+            sprintf(
+                "Invoice '%s' should be valid against MINIMUM schema.\nErrors: %s",
+                $description,
+                implode("\n", array_map(fn($e) => $e->message, $errors)),
+            ),
+        );
+    }
+
+    public function test_minimum_schema_does_not_validate_against_other_schemas(): void
+    {
+        $invoice = new CrossIndustryInvoice(
+            document: new ExchangedDocument('F0001', TypeCode::INVOICE, new DateTimeImmutable('2024-01-15')),
+            sellerTradeParty: new TradeParty('Seller', '12345678901234', CountryCode::FRANCE, new VATNumber('FR12345678901')),
+            buyerTradeParty: new TradeParty('Buyer', '98765432109876', CountryCode::FRANCE, new VATNumber('FR98765432109')),
+            monetarySummation: new MonetarySummation('EUR', 100.00, 120.00, 120.00),
+        );
+
+        $xml = $invoice->toXml();
+
+        $this->assertTrue($this->facturX->isValid($xml, Schema::MINIMUM), 'Should be valid for MINIMUM');
+        $this->assertFalse($this->facturX->isValid($xml, Schema::BASIC_WL), 'Should NOT be valid for BASIC_WL');
+        $this->assertFalse($this->facturX->isValid($xml, Schema::BASIC), 'Should NOT be valid for BASIC');
+        $this->assertFalse($this->facturX->isValid($xml, Schema::EN16931), 'Should NOT be valid for EN16931');
+        $this->assertFalse($this->facturX->isValid($xml, Schema::EXTENDED), 'Should NOT be valid for EXTENDED');
+    }
+}