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

what you can do is provide your own defined variable, and use a command design pattern to test the outcome for instance:

$.ajax({
   url: 'http://URL/test.php',
   data: {action: "ajax_request"},
   complete: function(res) {
   console.log(res.responseText);
 }
 });

and the php test:

if (isset($_POST['action']) && !empty($_POST['action'])) {
    $action = $_POST['action'];
    switch ($action) {
         case 'ajax_request' : echo 'This is an ajax request!';
        break;

   }
}
else
  echo 'This is not an ajax request!';

try this also

 while(true)
 {
     ......
    if (window.XMLHttpRequest){
       echo 'This is an ajax request!';
       return new XMLHttpRequest();
    }
     else if(window.ActiveXObject)// for internet explorer
    {
         echo 'This is an ajax request'!;
         return new ActiveXObject("Microsoft.XMLHTTP");
    }
    else
       echo 'This is not an ajax request!';
 }