Switching on PHP variables

[Tim Dawson]

There's a PHP variable "$_SERVER['HTTP_X_REQUESTED_WITH']", which can be used to detect Ajax requests. I understand it is only available on some servers. I hope, therefore, that it can be switched ON or OFF. It's certainly not listed with the PHP variables when I run 'phpinfo()' on XAMPP.

Can anyone tell me if this variable can be switched on somewhere, and if so, how, please ?