SSO $redirect_back_to_url

Question

I successfully implemented single sign on with CodeIgniter. Now, when users login using QA it redirects them to the main site login page. How do I redirect them back to where they came from?

I don't how to setup the $redirect_back_to_url inside qa-external-users.php. Is there somewhere else that the variable needs to be assigned?

When I do this :

^{'login' => $relative_url_prefix.'../ucp/login?redirect='.urlencode('http://mysite.com/qa/'.$redirect_back_to_url),}

at my login page I could see all that URL, but it doesn't redirect.