Redirect an url with utm query parameter and add additional parameter

[NicoHood]

Describe the bug
I have setup some redirections for having utm parameters hidden behind shorter links, like
https://example.com/partner-instagram redircts to https://example.com/?utm_medium=button&utm_source=instagram&utm_content=profil&utm_campaign=website-buttons.

Sadly some services like instagram will add some additional parameter to the url like this: https://example.com/partner-instagram?fbclid=PAZXh0bgNhZW0CMTEAAaf1-7aQ0LrYbkOoNOJqMK0VHOwSswZPCCNnVioFMNUGKTuE5I4mVnnaCIspVA_aem_0t4QVJk4xOGV0CX1pHasuA&fbclid=PAQ0xDSwLtljVleHRuA2FlbQIxMAABp3xqN3fubgEhDnTGlo5cOomSQD19oHgCisK9oNwS13UfbXL-zciGB9ryhP--_aem_rH2C-3SOrKao--xBK4GJcQ which then redircts to https://example.com/?utm_medium=button&utm_source=instagram&utm_content=profil&utm_campaign=website-buttons?fbclid=PAZXh0bgNhZW0CMTEAAaf1-7aQ0LrYbkOoNOJqMK0VHOwSswZPCCNnVioFMNUGKTuE5I4mVnnaCIspVA_aem_0t4QVJk4xOGV0CX1pHasuA&fbclid=PAQ0xDSwLtljVleHRuA2FlbQIxMAABp3xqN3fubgEhDnTGlo5cOomSQD19oHgCisK9oNwS13UfbXL-zciGB9ryhP--_aem_rH2C-3SOrKao--xBK4GJcQ

Which is a broken query parameter. We'd just need to merge the query parameters here. And its quite simple, the referring code is located here:

seo-by-rank-math/includes/modules/redirections/class-redirector.php

Line 139 in 9a198ad

$this->redirect_to .= '?' . $this->query_string;

To Reproduce
Steps to reproduce the behavior:

1. Add a redirect as mentioned above
2. Open the modified link as mentioned above
3. Check the not correct query parameter "merge" (no real merge happens, that is the problem)

Expected behavior
I would expect to merge the query parameters chaning the second ? to & in the code:
https://example.com/?utm_medium=button&utm_source=instagram&utm_content=profil&utm_campaign=website-buttons&fbclid=PAZXh0bgNhZW0CMTEAAaf1-7aQ0LrYbkOoNOJqMK0VHOwSswZPCCNnVioFMNUGKTuE5I4mVnnaCIspVA_aem_0t4QVJk4xOGV0CX1pHasuA&fbclid=PAQ0xDSwLtljVleHRuA2FlbQIxMAABp3xqN3fubgEhDnTGlo5cOomSQD19oHgCisK9oNwS13UfbXL-zciGB9ryhP--_aem_rH2C-3SOrKao--xBK4GJcQ

Additional context
Original code:

seo-by-rank-math/includes/modules/redirections/class-redirector.php

Line 139 in 9a198ad

$this->redirect_to .= '?' . $this->query_string;

if ( true === $this->do_filter( 'redirection/add_query_string', true ) && Str::is_non_empty( $this->query_string ) ) {
	$this->redirect_to .= '?' . $this->query_string;
}

Some code solution suggested by the ai (not tested):

if ( true === $this->do_filter( 'redirection/add_query_string', true ) && Str::is_non_empty( $this->query_string ) ) {
	if ( strpos( $this->redirect_to, '?' ) !== false ) {
		$this->redirect_to .= '&' . $this->query_string;
	} else {
		$this->redirect_to .= '?' . $this->query_string;
	}
}

And some more detailed code, which also supports fragments like #anchor. You might need to take a closer look, if that is correct (not tested)

if ( true === $this->do_filter( 'redirection/add_query_string', true ) && Str::is_non_empty( $this->query_string ) ) {

	// Parse the existing redirect URL
	$parsed_url = parse_url( $this->redirect_to );

	// Extract existing query parameters from the redirect URL (if any)
	$target_params = [];
	if ( isset( $parsed_url['query'] ) ) {
		parse_str( $parsed_url['query'], $target_params );
	}

	// Parse the incoming query string (e.g. from the original request, like fbclid)
	$new_params = [];
	parse_str( $this->query_string, $new_params );

	// Merge both sets of parameters (new ones can override existing ones)
	$merged_params = array_merge( $target_params, $new_params );

	// Rebuild the full query string
	$merged_query = http_build_query( $merged_params );

	// Rebuild the final redirect URL with merged parameters
	$this->redirect_to =
		( isset( $parsed_url['scheme'] ) ? $parsed_url['scheme'] . '://' : '' ) .
		( $parsed_url['host'] ?? '' ) .
		( $parsed_url['path'] ?? '' ) .
		'?' . $merged_query .
		( isset( $parsed_url['fragment'] ) ? '#' . $parsed_url['fragment'] : '' );
}