Will Drupal or PHP use APC if its installed on the server?
Drupal, like any other PHP app will an APC opcode cache if it's available.  Though there are a few settings that may prevent Drupal from using APC as it should.

First and foremost make sure APC has enough memory to work with.  Drupal can eat up a ton of memory and if your APC cache is too small it may actually hurt your performance.

The first memory relevant setting I would look at is the apc.shm_size.  APC's defaults setting have never worked out of the box for any of my Drupal installs. On some servers I have this is as high as 512M.

apc.shm_size = &lt;MB Allocated&gt;




APC may not be caching .module and .inc files by default! 

Depending on your setup, APC may not be caching .MODULE and .INC files at all. This means that the majority of Drupal's slowest code may never make it to the APC cache in the first place.

The fix?  apc.filters

On many of my Ubuntu servers adding the following line to my apc.ini has an enormous performance impact on my Drupal installs.

apc.filters = "+module$,+php$,+inc$,-admin"


You may also want to monitor your APC cache using the apc.php script that's generally bundled with APC.

You can also use apc.php to verify that .module and .inc files are making it into your APC system cache.