If you have APC set up on your server, you may see this error message when opening your WordPress dashboard:

Fatal error: Call to undefined function wp_dashboard_setup() in /xxx/wp-admin/index.php on line 15

This happens because of a configuration of your APC. You must have apc.include_once_override option set to 1. If you encounter the same problem, just set it to 0.

Well, that is not a recommended solution if your server only host few of your own WordPress websites, especially if those sites have huge traffic, since apc.include_once_override will improve your performance. So, to solve this without having to set apc.include_once_override to 0, you just need to edit wp-admin/index.php, change this code:

require_once(ABSPATH . 'wp-admin/includes/dashboard.php');

To:

require_once('./includes/dashboard.php');

Note : requiring local files with “./” makes you save some CPU time.