The following error may be found in /var/log/httpd/${domain}/error_log when APC memory is set too low: “PHP Warning: require_once(): Unable to allocate memory for pool”. APC is Alternative PHP Cache. You can check the version of php-pecl-apc, httpd, and php installed on your system by running the following command:

rpm -qa | egrep "httpd-[0-9]|php-pecl-apc|php-[0-9]"

To address the issue, edit the php.ini and add/modify the following line:

apc.shm_size = 256M

The amount of memory you need can be determined by using the apc.php that comes as part of the php-pecl-apc package. On RHEL/CentOS the file is located in /usr/share/doc/php-pecl-apc-${version}/apc.php. Copy that file to your httpd root and chown it apache:apache, or whatever uid:gid your httpd is using. Additionally, edit this copy of apc.php to set username and password:

defaults('ADMIN_USERNAME','apc');      // Admin Username
defaults('ADMIN_PASSWORD','your_password');  // Admin Password

After making the change, restart httpd (/sbin/service httpd restart) and check the apc.php page again to see if the apc.shm_size value has changed. If not, edit /etc/php.d/apc.ini an update the apc.shm_size variable there as well. Restart httpd again and verify.

You may not want to keep the apc.php file on your server: just use it for tuning your PHP settings and then remove it. If you want to keep apc.php on the server for future use, I’d recommend password-protecting it. Here are the basic steps:

1. Generate username/password

/usr/bin/htpasswd -bc /var/www/htdocs/${domain}/.htpasswd ${username} ${password}

2. Add the following lines to /var/www/htdocs/${domain}/.htaccess

AuthUserFile /data/htdocs/${domain}/.htpasswd
AuthName "Login to view APC INFO"
AuthType Basic
<Files "apc.php">
  require valid-user
</Files>

3. Reload httpd configuration

/sbin/service httpd reload