Update PSR-Cache.md

[RichardLynch]

Suppose you have a Drupal plugin that uses PSR-Cache and is configured to use, say, APC.

Then you add, say, a Symfony app that also uses APC.

There are bound to be key collisions here.

I would propose a "prefix" property and a composeKey ($key) { return $this->prefix . $key; }

This would allow each app to have their own configurable prefix providing a namespace.

It still doesn't guarantee two children won't use the same prefix . $key, but at least it gives the developers a sensible way out of the dilemma.

Frameworks could have standards to reduce the probability of collision. Ex, Drupal could standardize on 'Drupal_Modulename_site' as the default.

The 'site' part covers multisite.

[tedivm]

You can't have "private" or "protected" fields inside of an interface, it'll throw an error.

This type of detail is very, very important, but it falls more onto the implementing library to handle. We talked a bit about adding a "namespace" field into this version of the interface but it was rejected as too complicated by the group.