每次我加入php时都会注销

Question

I'm using http://www.php-login.net/ minimal as user management system. When I use $_GET to include the PHP page, I get logged out. This doesn't happen when I log in on my development machine. But when I tried on a different computer, I get logged out. How can I find out what's causing this?

Im guessing its my session?

Some part of my index.php

require_once("db/configure.php");

require_once("classes/Login.php");

include_once("analyticstracking.php");        
if ($login->isUserLoggedIn() == true) {

        include("inc/user.php");

    } else {

        include("inc/nonuser.php");
    }

My user.php

<?         
      $pageArray = [
     'home' => 'views/home.php',
     'setting' => 'views/setting.php',
     'article' => 'views/article.php'
 ];

 if(isset($_GET['page'])){
     $page = $_GET['page'];
 }else{
     $page = null;
 }
 if($page && isset($pageArray[$page]) && $_SESSION['user_id'] > 0){
     include $pageArray[$page];
 }else{
     include "views/home.php";
 }
 ?>