How do I get $redirect_back_to_url to work

Question

I am trying to integrate Q&A to my wiki site. 

I have set edited /qa-external/qa-external-users.php

and set  if (USE_LOGIN_LINKS)

    {

            return array(

'login' => '../index.php?title=Special:UserLogin redirect=".urlencode("qa/".$redirect_back_to_url)',

 

but how do I get $redirect_back_to_url to actually get the URL. At the moment it just says $redirect_back_to_url which gives an error as the page doesn't exsit. 

Thanks