Q:

PHP: $_SESSION doesn't seem to get set. Any idea?

I got through this SO Q/A while searching why my session wouldn't get set.
On $_GET set $_SESSION won't work
I don't really get whether this applies to my case. Here it goes.
index.php
<?php 
session_start();

if (!isset($_SESSION["authenticated"])) $_SESSION["authenticated"] = false; 
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html>
<head>
...

Then, I call the following when the user authenticates himself.
authenticate.php
<?php
require_once "data/data_access.php";

$userName = "";
$password = "";

if (isset($_REQUEST["userName"])) $userName = $_REQUEST["userName"];
if (isset($_REQUEST["password"])) $password = $_REQUEST["password"];

$isAuthentic = isAuthenticUser($userName, $password);
$_SESSION["authenticated"] = $isAuthentic;
echo $isAuthentic;
?>

I also have this authentication-thing checked every 5 seconds in order to create the buttons for the section the user is in.
authenticated.php
<?php echo isset($_SESSION["authenticated"]) && $_SESSION != false ? 'true' : 'false'; ?>

and my Ajax call setup:
my_project.js
$(document).ready(function() { startAuthenticationChecking(); });

function startAuthenticationChecking() { 
    setInterval(function() { ajaxSession("authenticated.php"); }, 5000);
}

function ajaxSession(actionURL) {
    var authenticated = false;
    $.ajax({
        async: false,
        url: actionURL,
        success: function(authenticated) {
            alert(authenticated);
            if (authenticated) {
                if (("#addNewAtvButtonDiv").is(":empty"))
                    $("#addNewAtvButtonDiv").add("<button id='newAtvButton'>Inscrire un nouveau VTT en inventaire</button>");
                if (("#addNewSledButtonDiv").is(":empty"))
                    $("#addNewSledButtonDiv").add("<button id='newSledButton'>Inscrire un nouvel UTT en inventaire</button>");
                if (("#addNewUtvButtonDiv").is(":empty"))
                    $("#addNewUtvButtonDiv").add("<button id='newUtvButton'>Inscrire une nouvelle motoneige</button>");
                $("button").button();
            } else {
                $("#addNewAtvButtonDiv").children().remove();
                $("#addNewSledButtonDiv").children().remove();
                $("#addNewUtvButtonDiv").children().remove();
            }
        }
    });
}

My Problem

Though I set $_SESSION["authenticated"] = false right after the session_start();, when the ajaxSession() asks whether a user is authenticated through authenticated.php, it always returns 'false'. Event after an authentication using authenticate.php with proper credentials.
Related question: PHP / Ajax : How to show/hide DIV on $_SESSION variable value?
Any help welcome! I've been working on this for a few nights and days lately, and still am.
Thanks!

A:

Make sure that all pages that use $_SESSION variables have session_start(); declared before any output.
It looks like authenticate.php doesnt have session_start(); declared, but uses a session request below.

$_SESSION["authenticated"] = $isAuthentic;

This will cause problems unless you have included/required the file authenticate.php, and the parent file has the session_start(); declared.
N.B:
session_start(); will either create a session or resume one. It is required on every page that uses session variables.
php.net

session_start() creates a session or resumes the current one based on a session identifier passed via a GET or POST request, or passed via a cookie. 

