Weird thing is, I read conflicting info about APC. Xenforo said it wasn't compatible, and I read it had been deprecated and no longer supported since there is some built in cache mechanism in PHP now.
If you have APC (Alternative PHP Cache) installed on your server, you may use it to store cache data.
Note that some versions of APC may be susceptible to cache-slam issues, so if you do enable APC caching of data (in addition to the PHP opcode cache that it maintains automatically without requiring any configuration), you should monitor your server carefully for a time to ensure that it is running smoothly.
Memcached and APC do not do the same things. APC is primarily an opcode cache.
APC is a third party module that is important to achieve high performance in PHP 5.4 and earlier versions. While there are versions of APC that work with PHP 5.5, it also has a built-in opcode cache that is meant to replace the third-party functionality. PHP 5.5 is installed on our webhosting machines but currently has to remain disabled because Zend Guard has not come out with a new version that is compatible with it.
