getting oscommerce's login variable on shopping_cart.php page

[PHP_User]

I am trying to find a way to look up a users name and password (passed in the url with get) so I can log them in after hitting checkout at the site. (they are logged into my main site with $_SESSION variables),

 

I don't want to force them to log in again and I have been trying to take the log in code from the login.pho page and re-use it on the shopping_cart.php page.

 

I can keep my $_SESSION variable on these pages, and can pass the username and passwords with a GET, but cannot seem to look them up before completing their purchase and making them log in after hitting check out.

 

Thanks,

Don