It would be a lot more effective/abstract to pass a relative URL to the login script.
Upon loading, the script checks if they are logged in or not. If they aren't, forward to login page (appending the current relative URL to the forward).
Then the login page uses that to forward back to the original page.
Login Forward: www.domain.com/login.php?returnURL=newsletter/ (this needs to be urlencode()d).
This way you can reuse the code for the login page.