The global PHP built-in variable $_SERVER is not set if a
page is produced from the cache. If the PHP script was
recently modified and/or there is no cached version
available, it works fine. Other global built-in variables
($_POST, $_GET etc) are always accessible.
I have currently only tested this with MMCache 2.4.6
using PHP 5.0.0b3, Apache 2.0.47 (worker-mpm,
threaded) and Linux 2.6.1.