APC or Alternative PHP Cache (APC) is a free and open opcode cache for PHP. APC allows for caching of opcode generated during PHP script parsing and execution by the Zend engine. If you are using APC 3.1.9 and you are using the default configuration you might run into the error where most PHP requests will return a warning message "PHP Warning: Unknown: Unable to allocate memory for pool. in Unknown on line 0"