PHP session being lost - php

Question

I'm trying to implement and authentication system with jQuery and PHP. All the php work is made in the controller and datahandler class. There is no php code inside the .html files, all the values in .html files are rendered via jQuery that request the data from php server. So what I'm trying to do is:
When user clicks the login button, the jQuery makes a call to the authenticate() method in my controller class, it checks if the user is correct and stuff, and if it is, start the session and set the user_id on the session so I can access it later, and returns the userId to the jQuery client again.
After that, if everything is fine, in jQuery I redirect it to the html file. On the html file I call a jQuery from the <script> tag that will handle other permissions. But this jQuery will access the method getPermissionString (from the same class of authenticate() method mentioned before), and it will need to get the session value set in authenticate method.
The Problem:
When I try to get the session value inside getPermissionString() it says:
Notice: Undefined variable: _SESSION
I've tried to check if the session is registered in the second method, but looks like it's not. Here is my PHP code.
Any idea? Thanks.
public function authenticate($login, $password)
{
$result = $this->userDataHandler->authenticateUser($login, $password);
if(is_numeric($result) && $result != 0)
{
session_start();
$_SESSION["uid"] = $result;
if(isset($_SESSION["uid"]))
{
echo "registered";
$userId = $_SESSION["uid"];
}
else
{
echo "not registered";
}
echo $result;
}
else
{
echo 0;
}
}
public function getPermissionString()
{
if(isset($_SESSION["uid"]))
{
echo "registered";
$userId = $_SESSION["uid"];
}
else
{
echo "not registered";
}
}

Answer 1

Before you can access $_SESSION in the second function you need to ensure that the program has called session_start() beforehand. The global variable is only populated when the session has been activated. If you never remember to start a session before using it then you can change the php.ini variable below:
[session]
session.auto_start = 1
Further, you said that you're using a class for your code. In this case you can also autos tart your session each time the class in created by using magic methods:
class auth {
function __construct() {
session_start();
}
function yourfunction() {
...
}
function yoursecondfunction(){
...
}
}

Answer 2

If you don't have session.auto_start enabled, and authenticate and getPermissionString are called on two different requests, you need to call session_start() in each function.
If you need more information on how the session ID is passed, just read Passing the Session ID

Answer 3

You should not use that function if session is not started. So throw an exception:
public function getPermissionString()
{
if (session_status() !== PHP_SESSION_ACTIVE)
{
throw new Exception('No active session found.');
}
if(isset($_SESSION["uid"]))
{
echo "registered";
$userId = $_SESSION["uid"];
}
else
{
echo "not registered";
}
}
This ensures the pre-conditions of your functions are checked inside the function so you don't need to check it each time before calling the function.
You will now see an exception if you wrongly use that function and it will give you a backtrace so you can more easily analyze your code.

Answer 4

For php sessions to work you have to call session_start() every time you script is requested by the browser.