How to find out if a request is an ajax request?

Question

I try to find out if a request to a PHP file is sent by ajax or not.

I googled it and read a whole a bunch of articles that suggest following method:

Answer 1

Not all browsers will send that response I usually use

if (isset($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) === 'xmlhttprequest') {//Do stuff }

and for ajax

request = $.ajax({
    url: SomePage.php,
    type: "POST",
    data: {key: value}
});
request.done(function(returnedData) {
    //do done stuff
});
request.fail(function(jqXHR, textStatus) {
    //do fail stuff
});

Note:

$HTTP_SERVER_VARS contains the same initial information, but is not a superglobal. 
(Note that $HTTP_SERVER_VARS and $_SERVER are different variables and that 
PHP handles them as such). Also note that long arrays were removed since PHP 5.4.0 so 
$HTTP_SERVER_VARS doesn't exist anymore.

So var_dump($HTTP_SERVER_VARS); to see if its contained in there, also note that the $_SERVER is filled in by the webserver