Description:
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In the documentation for apc_store() and apc_add(), the 
circumstances for which an entry is removed from the cache is 
not fully explained when ttl=0.  The docs list "clear, 
restart, etc", and it is not clear what "etc" covers.

It is also unclear what happens when the cache is full, and 
apc_store()/apc_add() is called, and whether LRU, LFU, or 
another cache strategy is employed to free up space.

The documentation for the apc.ttl option hints that no other 
cache strategies are used, but it could be more explicit.