I am creating a signup form with a certain number of open slots. I would like to create a timeout/refresh page overlay window if there is no activity for 5 minutes so that people can't let the page sit for an hour and then sign up for a position that has already been taken while they were waiting. Ideally, if they wait too long a translucent window will overlay with a button that will refresh the page and restart the session.

I think that I can use the following in some way:

PHP Code:
<?php

session_start();
 
// set timeout period in seconds
$inactive = 300;
 
// check to see if $_SESSION['timeout'] is set
if(isset($_SESSION['timeout']) ) {
    //$("timeoutMessage").style.display = "none";
    $session_life = time() - $_SESSION['timeout'];
    if($session_life > $inactive) { 
        session_destroy(); 
        //SOMETHING HAPPENS HERE I THINK
    }
}
$_SESSION['timeout'] = time();

?>
But I can't figure out how to trigger a div/overlay without manually refreshing the browser. I've seen many, many sites do this but for reason I can't find any examples. I'm not even sure what to call it when I do a search. Can anyone just point me in the right direction?

Thanks.