<?php

/**
* Somewere (on login.php for example) set $_SESSION['SessionExpireTime'] = time() + 1200;
*/
?>

/**
* Then on every page or layout file do
*/
<?php

session_start();
if($_SESSION['SessionExpireTime'] > time()){
    $_SESSION['SessionExpireTime'] = time() + 1200; // 20 minutes
    session_regenerate_id();
}else{
    HEADER('location:admin_login.php');
}


?>