How do I use sessions in PHP and Constrcut's AJAX?

<?php
	header('Access-Control-Allow-Origin: *');
    session_start();

    if(isset($_SESSION)){
        echo "Session variable exists<br/>";

        if(!isset($_SESSION['test'])){
            $_SESSION['test'] = "Success!";
            echo "Variable has been set, refresh the page and see if stored it properly.";
        }else{
            echo $_SESSION['test'];
        }
    }else{
        echo "No session variable has been created.";
    }
?>
[/code:x3hft4n9]

When I run this script in my browser it works perfectly:
[ul]
[li] First: "Variable has been set"[/li]
[li] Refresh: "Success!"[/li][/ul]

But when I create a button in Construct 2 which sends an AJAX request to that session_test.php script the following happens:
[ul]
[li] First: "Variable has been set"[/li]
[li] Refresh: "Variable has been set"[/li][/ul]

It seems like there's always recreated a new session. I also tried to export my game and to upload it onto my webserver - same result. Does it have something to do with Construct 2?