倒数计时器r在零时从24小时开始计时

Question

I developed a computer based testing system where students can test their knowledge with countdown timer. I used the following codes which relate to the server very well.

first_page.php

<?php
    session_start();
    $get_time="select * from tests";
    $run_time = mysqli_query($con, $get_time);
    $row_time=mysqli_fetch_array($run_time);
    $duration=$row_time['duration'];

    $_SESSION["duration"]=$duration;
    $_SESSION["start_time"]=date("Y-m-d H:i:s");
    $end_time=date('Y-m-d H:i:s', strtotime('+'.$_SESSION['duration'].'minutes', strtotime($_SESSION['start_time'])));
    $_SESSION['end_time']=$end_time
?>

<script type="text/javascript">
    window.location="second_page.php";
</script>

second_page.php

<div id="response"></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);
</script>

response.php

session_start();
$from_time1=date('Y-m-d H:i:s');
$to_time1=$_SESSION['end_time'];

$timefirst=strtotime($from_time1);
$timesecond=strtotime($to_time1);
$differenceinseconds=$timesecond-$timefirst;
echo gmdate("H:i:s",$differenceinseconds);

The problem now is that when the timer gets to zero, it goes to 24hrs and start counting down. Is there a way to amend these codes so that when the timer gets to zero it stops and redirects the user to a different page where I can say “You have used up your time and your answers have been submitted automatically”?

Answer 1

You could do a check in your response callback, something like this:

if (xmlhttp !== '0 minutes') {
    document.getElementById("response").innerHTML=xmlhttp.responseText;
} else {
    window.location = 'put your url here';
}

When looking at your code i saw you do a request every 1 second. That is really bad practise, the server load with multiple clients will be enormous. Cant you return a date with PHP and do the countdown in javascript? And re-check every 5 minutes if javascript is still in sync with the date from php? from (60 * 60 * 24) request you go to (20 * 24) request per day per client.