index.php

<?php

// include the configs / constants for the database connection
require_once("config/db.php");

// load the login class
require_once("classes/Login.php");

// create a login object. when this object is created, it will do all login/logout stuff automatically
// so this single line handles the entire login process. in consequence, you can simply ...
$login = new Login();

// ... ask if we are logged in here:
if ($login->isUserLoggedIn() == true) {
    // the user is logged in. you can do whatever you want here.
    // for demonstration purposes, we simply show the "you are logged in" view.
    include("logged_in.php");

} else {
    // the user is not logged in. you can do whatever you want here.
    // for demonstration purposes, we simply show the "you are not logged in" view.
    include("not_logged_in.php");
}