PHP unset session not working

My PHP script to unset session variable is not working.

I need to call a PHP file destroy.php which contains code to unset session variable from ajax, everything is working fine except session clear.

I'm posting my code for clearing session variable, please point out my error.

jQuery Code:

$.ajax
                    ({
                        type: "POST",
                        url: "SubmitReview.php",
                        data: dataString,
                        cache: false,

                        success: function(response)
                        {
                            $.fn.destroySession();
                            $('body').animate({scrollTop:0}, 500);
                            $('body').children(':not(div.ErrorMessageContainer)').fadeTo(500, 0.1);
                            $('div.ErrorMessageContainer').html(response).fadeIn(500).delay(5000).fadeOut(500);
                            setTimeout(function()
                            {
                                $('body').children(':not(div.ErrorMessageContainer)').fadeTo(500, 1);
                            }, 5400);
                            setTimeout(function()
                            {
                                window.location.href = "Thanks.php";
                            }, 5600);
                        }
                    });

$.fn.destroySession = function()
            {
                $.post("Destroy.php");
            }

destroy.php:

<?php

if(isset($_SESSION['MN']))
{
    $_SESSION['MN'] = array();
    unset($_SESSION['MN']);
}

?>