如何在php中启用javascript启用的倒计时器？

I have written php code for an examination portal using javascript enabled countdown timer. When i run my code in localhost its fine. But in server, the clock gets stopped after some time, what might be the problem? Below is the part of related codes. Thank you!!! In portal.php

    <?php
    $_SESSION['exam_length'] = $all_questions[0]->exam_length;
    $_SESSION['start_time'] = date('Y-m-d H:i:s');
    $end_time = date('Y-m-d H:i:s',strtotime('+'.$_SESSION['exam_length'].'minutes',strtotime($_SESSION['start_time'])));
    $_SESSION['end_time'] = $end_time;
    ?>
    <div  class="col-md-3 col-lg-3 col-sm-3" id="response" style="font-size: 25px;  position: fixed; right: -7px;"></div>

    <script type="text/javascript">
    setInterval(function()
    {
        var xmlhttp = new XMLHttpRequest();
        xmlhttp.open("GET","response.php",false);
        xmlhttp.send(null);
        document.getElementById('response').innerHTML=xmlhttp.responseText;
    },1000);


    exam_length =  '<?php echo (($all_questions[0]->exam_length)*60000); ?>';
    var auto_refresh = setInterval(function() { submitform(); },exam_length);

   </script>

In response.php:

<?php 
     session_start();
    if(($_SESSION['is_logged_in'])=="")
    {
        @header("location:../");
        exit;
    }
    $from_time1 = date('Y-m-d H:i:s');
    $to_time1 = $_SESSION['end_time'];
    $timefirst = strtotime($from_time1);
    $timesecond = strtotime($to_time1);
    $difference_second = $timesecond - $timefirst;
    echo gmdate("H:i:s",$difference_second);
    ?>