* This is the implementation of the method declared in the parent class.
* @param string a unique key identifying the cached value
* @return string the value stored in cache, false if the value is not in the cache or expired.
*
* By definition, cache does not ensure the existence of a value
* even if it never expires. Cache is not meant to be an persistent storage.
*
* To use this module, the eAccelerator PHP extension must be loaded and enabled