There's also lots of very helpful information on this Moodle Docs page...
http://docs.moodle.org/23/en/Installing_APC_in_Windows
Even though the first portion of the page focuses on Windows servers, the later portions are applicable to Linux environments or any Moodle installation.
In particular, the sections on "Monitoring and tuning the [APC] cache" and "APC.php security" cover how to securely install and access from within Moodle the APC Opcode Cache's own cache administration scripts which let the Moodle admin monitor and even clear the APC cache in real time. Very useful in tuning the APC cache settings for your own server's needs.
http://docs.moodle.org/23/en/Installing_APC_in_Windows#Monitoring_and_tuning_the_cache