PHP: Require_once(): Unable To Allocate Memory For Pool Error and Solution

I'm running PHP version 5.3.3 on CentOS / RHEL 6.x with APC php-pecl-apc-3.1.9. However, I'm getting the following warnings in my php server log file:

[01-Jul-2012 10:50:49] PHP Warning: require_once(): Unable to allocate memory for pool. in /var/www/html/includes/admin.php on line 57 [18-Jul-2012 17:00:08] PHP Warning: include_once(): Unable to allocate memory for pool. in //var/www/html/includes/xmlrpc.php on line 55 [18-Jul-2012 17:00:40] PHP Warning: require(): Unable to allocate memory for pool. in /var/www/html/includes/geshi.php on line 2307

How do I fix these php warnings?

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