PHP Session Time out- Auto Logout

<?php
session_start();
ob_start();
//Include the database connection file
include "database_connection.php"; 
//Check to be sure that a valid session has been created
if(isset($_SESSION["VALID_USER_ID"]))
{

    //Check the database table for the logged in user information
    $check_user_details = mysql_query("select * from `signup_and_login_users_table` where `username` = '".mysql_real_escape_string($_SESSION["VALID_USER_ID"])."'");
    //Validate created session
    if(mysql_num_rows($check_user_details) < 1)
    {
        session_unset();
        session_destroy();
        header("location: login.php");
    }
    else
    {

    //Get all the logged in user information from the database users table
    $get_user_details = mysql_fetch_array($check_user_details);

    $user_id = strip_tags($get_user_details['id']);
    $fullname = strip_tags($get_user_details['fullname']);
    $username = strip_tags($get_user_details['username']);
    $email = strip_tags($get_user_details['email']);
    $passwd = strip_tags($get_user_details['password']);
    $user_access_level = strip_tags($get_user_details['user_level']
    );
?>