访问页面的用户身份验证

I'm working on page authentication. It can login already, but I want it to make user authentication on other pages a swell if someone tries to access pages through URL. If the person is not a logged in user, redirect that person to the login page. I tried it by working with sessions but it doesn't work. I'm following MVC structure

Here is how I did it

My loginController

    <?php
//LoginController
if($_POST)
{
    if(isset($_POST['submit']) AND $_POST['submit'] == "login")
    {
        $username = $_POST['username'];
        $password = $_POST['password'];
        try
        {
            include '../model/Login.php';
            $login = new Login($db, $username, $password);

            if($login == TRUE)
            {
                session_start();
                $_SESSION['username'] = $username;
                header("Location:../index.php");
            }

        }
        catch (Exception $exc)
        {
            echo $exc->getMessage();
        }

    }
}

My index controller( for main page)

<?php
include 'model/Database.php';
session_start();
//Checks if the user is logged in.
if(!isset($_SESSION['username'])) {
//echo"<h2>You have no access to this page!";
    include 'view/login.php';
    die();
}
include 'c:/wamp/www/mvc/model/Display.php';

$displayPatients = new Display($db);
$dataDisplay = $displayPatients->getData();
include 'view/Main.php';
?>