Facebook PHP-SDK + jQuery AJAX

Im creating a Facebook app using PHP-SDK.

In order to find a user's uid I do:

index.php file

$user_id = $facebook->getUser();
if($user_id == 0) // not logged in user
  {
    // do stuff to log in user and get the current user id
  }开发者_Python百科

Inside index.php file I have the following jQuery code:

   var get_url =
      'do_stuff.php?id=' + <?php echo $user_id; ?> + '&field1=' + field1 + '&field2='+ field2;

       $.get(get_url, function(data) {
          alert("data: " + data);
       });

do_stuff.php inserts data on my MySQL database based on field1 and field2. I want to be sure that the data inserted are really from user with $user_id. And not just somebody who did a fake request like:

http://myapp.com/do_stuff.php?id=fake_user_id&field1=...&field2=...

So, I added the $user_id = $facebook->getUser(); at do_stuff.php but instead of returning the current user id (which is visible at index.php), it returns 0 which means no user is logged in. Why is this?

How I can fix it? So I will be sure that the data inserted on my MySQL database are from a real user and not a fake one?

This is because as far as do_stuff.php is concerned, no one is logged in. You probably need to look at capturing the access token and using that in do_stuff.php.