This is a 'superglobal', or automatic global, variable. This simply means that it is available in all scopes throughout a script. You don't need to do a global $_SERVER; to access it within functions or methods, as you do with $HTTP_SERVER_VARS.
function func()
{
var_dump($_GET); //array(0) {}
var_dump($HTTP_GET_VARS); // NULL
}
func();
register_long_arrays boolean
Tells PHP whether or not to register the deprecated long $HTTP_*_VARS type predefined variables. When On (default), long predefined PHP variables like $HTTP_GET_VARS will be defined. If you're not using them, it's recommended to turn them off, for performance reasons. Instead, use the superglobal arrays, like $_GET.
This directive became available in PHP 5.0.0.