未定义的变量：_SESSION在重定向的PHP文件中

Question

I am quite new to PHP. I am trying to write a login script in PHP. My index.html file takes the username and password and submits the form to login.php. This login.php file checks if the user is registered and creates a session if so. After that it redirects to the file controlpanel.php

My problem is I get proper values on reading the session variable in login.php(just before redirecting to controlpanel.php). However in controlpanel.php, when I try to read the session variable it gives the error:

( ! ) Notice: Undefined variable: _SESSION in D:\\Work\\Projects\\phpapp\\myecom\\admin\\controlpanel.php on line 2

My code:

login.php

<?php if(!isset($_SESSION)){ob_start(); session_start(); }else{error_log("Session already set");}
require_once("../config/config.php");
?>

<?php   
  require_once("../classes/autoload.php");
  $objUser = new User();

    if (isset($_POST["username"])){
        $username = $_POST["username"];
        $pass = $_POST["password"]; 

        error_log("about to check for registration");

        if($objUser->isRegisteredUser($username, $pass, 2)){
            error_log("user registered");

            Login::loginUser($objUser->id, 2);

            error_log("session id in login.php: " . $_SESSION["uId"]);

            header("Location:controlpanel.php");   
          }
        else{
            error_log("not registered");
          //redirect to the registration page
        }


    }


?>

controlpanel.php

<?php 
if(!isset($_SESSION)){
    error_log("session not set");
}else{
    error_log("session set in controlpanel: " . $_SESSION["uId"]);
}

?>

PHP error_log:

[25-Oct-2014 18:08:51 Europe/Paris] about to check for registration

[25-Oct-2014 18:08:51 Europe/Paris] user registered

[25-Oct-2014 18:08:51 Europe/Paris] session id in login.php: 1

[25-Oct-2014 18:08:51 Europe/Paris] session not set

Answer 1

You didn't start the session in your controlpanel.php script.

Always call session_start(); before using $_SESSION .