Countdown variable across many pages

I have a demo website for an e-test where each student is allowed 15 minutes for the whole exam (which comprises 5 subjects, each with 10 questions). I want it that, immediately a student clicks START on the home page, a countdown timer should start and update itself automatically. Actually, that is easily do开发者_StackOverflow社区ne for a single page.

The problem is if the student clicks to go the next page, I want the timer to begin from where it stopped on the previous page and continue counting down.

I thought of using SESSIONS but then how do I get Javascript to set the SESSION variable in PHP? The idea was to use AJAX but I do not know how to go about it.

Can anyone help me on it, even if it requires something other than AJAX?

PS:

What I want displayed is the number of seconds left, not the current time.

Thanks.

You don't have to know where it stopped on the other page.

You just want to know when the users clicked start.

Just save that in a database or a file even.

And on every request calculate the time left on the SERVER.

Since the client can be easily manipulated.

With the time left calculated on the server you can make a countdown using javascript on every page.

I found i simple way to do it, with a small search. Here

PHP:

//when you start
$_SESSION['start_time'] = time();

Then on every page:

<script type="text/javascript">
    var startTime = <?php echo $_SESSION['start_time']; ?>;
    //calculate remaining time
</script>

You can use an algorithm like this:

1. When the "start" event occurs, store the start time in a database.
2. Periodically send AJAX requests to a backend script. If the time has expired, return a response that ends the test.

There is not a real "timer" in this case -- just a start time, and logic to check if 15 minutes have passed since that time. If you want to display a timer to the user, the math will be straightforward.

You could just put the pages in IFrames and the countdown timer on the main page.

Simple, you store the timestamp of when the student first started the test and output it to each page. Then it's a simple matter of substracting current time from that timestamp to get elapsed time.

However, depending on Javascript is a very bad way of ensuring that this timer gets started. A better method is to force the user to click on an actual link/button, WITHOUT involving Javascript. You record the timestamp on the server then that hit comes in, and after that everything's fine.

start.php:

<a href="page1.php">Start Test - Page 1</a>

page1.php:

<?php

if(!isset($_SESSION['test_start_timestamp')) {
    $_SESSION['test_start_timestamp'] = time();
}
?>

<h3>Question #1</h3>
etc...

The important thing is to never reset the timestamp value in the session once it's been set. Otherwise the test taker can simply click around until they reach a page where the counter's reset, and they can get more time for the test.