<?php 
     session_start(); 
     $_SESSION['session_time'] = time(); //got the login time for user in second 
     $session_logout = 900; //it means 15 minutes. 
     //and then cek the time session 
    if($session_logout >= $_SESSION('session_time']){ 
        //user session time is up 
       //destroy the session 
      session_destroy(); 
     //redirect to login page 
     header("Location:the-path-your-login-page.php"); 
    } 
?>