Using $_GET to retrieve mycustomvariable
===

I am trying to use the $_GET operator in php to get mycusomvariable from an fb app url to try to append a username, and find that this appears to return no details?

As an example here is the url I'm posting to Facebook:

**http://apps.facebook.com/my-app-namespace?username=Test**

and here is the php code which should retrieve the string 'Test'

        <?php

          $username = $_GET ['username'];
          echo $username;

        ?>



This should show up in the browser as Test, but shows nothing.

Anyone any ideas on where I'm going wrong?