The apc.stat = Off setting in APC tells PHP to ignore all file includes and requires. Improper usage of this option often leads to white pages or cryptic and wierd PHP exceptions. What this setting does is on the first run, APC stores looks for and stores all includes/requires declared and on the next request, PHP will ignore all file includes/requires. Okay, so now your thinking that this is simple, but the catch is that all includes/requires must be absolute (from /). Because most, perhaps every library does not use absolute paths, generally for most people, this setting will cause problems; however, if you use a script to replace the relative paths with absolute paths, a performance improvement of up to the APC advertised 30% can be achieved.