无法通过AJAX设置PHP会话

Last 4 hours passed with my effort to make a basic setting of a PHP session.

Here's what I have,

I have index.html that has a javascript which contains an on click function as;

$("a.yes").on('click', function() {

    $.ajax({

            type: "POST",
            url: "http://www.example.com/blabla/php/register.php",
            data: "test=" + "test2"

        })
          .done(function(data){

        alert($(data).filter(".meme").text());
        })
        .fail(function() {

            alert( "Error" );

        });
});

My AJAX Post handler file is (aka register.php);

<?php
session_start();
header('Access-Control-Allow-Origin: *'); 
include "../../includes/connect.php";

if ($_POST['test']){
    $_SESSION['mysession'] = $_POST['test']
?>
<span class='meme'><?=$_SESSION['mysession']?></span>
<?php

}
?>

It alerts my posted value as test2 but not setting the session when I check it afterwards. But if it is not setting the session how it alerts test2?

What could be wrong? PS: I tried without header('Access-Control-Allow-Origin: *'); but result is the same. I need it anyway.

UPDATE: It seems sessions for www and non www version are different... There is no problem when they are both www or non www. Solved.