Automatic Logout after 15 minutes of inactive in php

I want to destroy session if users are not doing any kind of activity on website. At that time after 5 users automatically redirect on index page. How is it possible? Is possible in php with session handling and for that I have to maintain or update user login time or not..

Answer

This is relatively easy to achive with this small snippet here:

 if(time() - $_SESSION['timestamp'] > 900) { //subtract new timestamp from the old one
    echo"<script>alert('15 Minutes over!');</script>";
    unset($_SESSION['username'], $_SESSION['password'], $_SESSION['timestamp']);
    $_SESSION['logged_in'] = false;
    header("Location: " . index.php); //redirect to index.php
    exit;
} else {
    $_SESSION['timestamp'] = time(); //set new timestamp
}