Login Page not redirecting on session active

Question

I've done a login page in a site for some company and every seemed to be alright until I notice a minor issue, important anyway. The session starts an everything works ok, except that I can't redirect from the login page if the session is already started. I mean if I type the address:

mydomain/myfolder/login.php

It stays there and can't redirect to the main page, because it looks like the session data is not available to this page. Here's the code I'm using:

<?php

$usuarios = simplexml_load_file('__usuarios__mensajes.xml');
$goLogin = false;
$nameOK = false;
$passOK = false;
$messagesPage = "index.php";

if($_SESSION["loggedIn"])$passOK = true;

if((isset($_POST["MM_insert"]) && $_POST["MM_insert"] == "form1")){
    $nombre =  $_POST["user"];
    $password = $_POST["password"];

    foreach($usuarios->usuario as $usuario){
        $attr = $usuario->attributes();

        if($nombre == $attr["nombre"]){
            $nameOK = true;
            if($password == $attr["password"]){

                $goLogin = true;
                $passOK = true;
                $displayName = (string)$attr["displayName"];

                }               
                break;
            }
        }
    }

    if($passOK){
                session_start();

                $_SESSION["usuario"]= $_POST["user"];
                $_SESSION["loggedIn"] = true;
                $_SESSION["userDisplayName"]= $displayName;                 

                header(sprintf("Location: %s", $messagesPage));

            }
?>

Answer 1

Before you use $_SESSION , use session_start() first. So it should be like this:

session_start();
if($_SESSION["loggedIn"])$passOK = true;

Answer 2

you have to check for session validation fil first line after

 if(!isset($_SESSION)){ session_start(); }