APC was conceived of to provide a way of boosting the performance of PHP on
heavily loaded sites by providing a way for scripts to be cached in a compiled
state, so that the overhead of parsing and compiling can be almost completely
eliminated. There are commercial products which provide this functionality, but
they are neither open-source nor free. Our goal was to level the playing field
by providing an implementation that allows greater flexibility and is
universally accessible.

NOTE!: php-apc has to be loaded last, very important!

This package comes with four different flavours of APC (use only one of them):

o apc-mmap.so - mmap (fcntl) based locks (default)
o apc-sem.so - IPC semamphore based locks
o apc-spinlocks.so - Hardware-dependent implementation of spinlocks
o apc-pthread.so - NPTL pthread mutex based locks
o apc-mmap+mutex.so - mmap (fcntl) and pthread mutex based locks