function is_session_valid()
{
    $check_ip = true;
    $check_user_agent = true;
    $check_last_login = true;
    if ($check_ip && !request_ip_matches_session()) {
        after_successful_logout();
        return false;
    }
    if ($check_user_agent && !request_user_agent_matches_session()) {
        after_successful_logout();
        return false;
    }
    if ($check_last_login && !last_login_is_recent()) {
        after_successful_logout();
        return false;
    }
    return true;
}
<?php

/* ================================================================ */
/* Created by Engin Yapici on 10/19/2015                            */
/* Last modified by Engin Yapici on 10/19/2015                      */
/* Copyright Engin Yapici, 2015.                                    */
/* enginyapici@gmail.com                                            */
/* ================================================================ */
// Below if statements prevents direct access to the file. It can only be accessed through "AJAX".
if ($_SERVER['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest') {
    require '../../private/include/include.php';
    after_successful_logout();
    echo 'success';
} else {
    echo "Direct access is not permitted";
}