PHP 5.4 came with APC (Alternative PHP Caching) which acted as both a byte-code cache (reducing the work of compiling the PHP files at runtime) and an in-memory object cache allowing the PHP app to store and retrieve arbitrary data.
PHP 5.5 is moving to Zend’s OpCache (bundled, but not enabled by default) which will replace APC’s function as a byte code cache. The gotcha is that OpCache does not (yet?) provide object caching that your app can use to store its own data.
So if you’re moving to PHP 5.5 you may find a bit of a black spot when it comes to an automatically included memory cache. APCu looks to provide the object caching of APC, but it does require extra installation.