如何获取加载页面后创建的php var值？

Question

首先加载此页面test1.php，然后点击CREATE VAR（var $_SESSION["test_var"]将创建），然后点击ALERT VAR，我想知道如何才能获得警报变量结果再次加载页面

test1.php

<?PHP
session_start();
include("connect.php");
$sessionid = session_id();
?>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>

<div onclick="alert_fn()">ALERT VAR</div>
<br><br><br><br>
<script>
function alert_fn(){
    alert("<?PHP echo $_SESSION["test_var"]; ?>");
}
</script>

<div onclick="create_var_fn()">CREATE VAR</div>
<br><br><br><br>
<div id="xx_span"></div>    


<form id="fid" method="post" action="" ENCTYPE = "multipart/form-data">
<input type="hidden" name="xx" id="xx">
</form>

<script>
function create_var_fn(){
    $.ajax
    (
        {
            url: 'test2.php',
            type: 'POST',
            data: $('#fid').serialize(),
            cache: false,
            success: function (data) {
                $('#xx_span').show();
                $('#xx_span').html(data);
            }
        }
    )
}
</script>

test2.php

<?PHP
session_start();
$_SESSION["test_var"] = "test";
echo $_SESSION["test_var"];
?>