login destination in combination with single sign on module

I believe this is the proper location for my challenging issue. The login destination is great module and it works like a charm in itself. However I am facing problems with the setup I am planning.

I have one installation in a multisite setup using the domain access module running only on one database. This module is used for 8 sites in total.To get Single Sign On (SSO) to work I had to create a site separate from the domain access sites with it's own database.

SSO works with a controller site which serves several client sites (7). The procedure goes basically like this:

you log in on one of the 7 sites available. this login session is being sent to the separate controller site which makes sure that any other login to one of the other 7 sites becomes unnecessary.

The 7 sites contain an ubercart webshop where anonymous users have to login before they can order products. Now when they login on the http://site1.com/shop page they are being redirected back to the homepage http://site1.com.

What I want is that they remain within the http://site1.com/shop after they have logged in. The login destination module in theory would be perfect for me to use. In order to get this module to work I in fact have to install it on the separate controller site (I hope you're still with me). Reason for this is because the login session on http://site1.com/shop is being sent to the separate controller site to make SSO work (as I explained earlier).

When configuring (now we arrive at my problem) I have to fill in a destination at 'Destination URL settings'. This somehow needs to be a php snippet (if possible) that redirects the user back to http://site1.com/shop. Whenever someone logs in on http://site2.com/shop though this PHP snippet needs to redirect the user to http://site2.com/shop and not back to http://site1.com/shop.

My question is: what would this php snippet look like? And if my problem cannot be solved by a php snippet, is there another way somehow.

There it is, a bit long maybe, but I wanted to state my problem accurately. Hopefully there is someone around here who can guide me to a solution or can point me in the right direction.