Automatic Logout after 15 minutes of inactive in php

I got this solution from Sitepoint.com Using a simple meta tag in your html

<meta http-equiv="refresh" content="900;url=logout.php" />

The 900 is the time in seconds that you want the session to be terminated if inactive.

Hope it works for you

Edit: This method does not implement any other logic so will only work if you want to "force" logout as said in the comments

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
}