function CheckAuthentication(), need some quick advice.

Hi all, I created a file called sessions.php with the following inside:

<?php
session_start();
print_r($_SESSION);
?>

Once I logged into my site, I ran the script and the following was returned:

[twg_admin_logged_in] => true

So can I tried to insert this into my config.php but it still does ot work, why?

function CheckAuthentication()
{
   if ( isset($_SESSION['twg_admin_logged_in']) )
      return true;
   else
      return false;
}