Saving PHP session variables between domains and https/http

I have a checkout form that posts to authorize.net to approve a transaction and then displays a receipt page back on my server. However, session variables set on the checkout page are not carried over to the receipt page. How c开发者_开发问答an I save session variables when the user is redirected to an external https domain and then back?

If I save the variables to a database, how do I even identify which data is associated with the user if the session variable with the username is lost!?

I had a similar problem with session not working just on one browser. Turns out I apparently had too many cookies for that host, clearing them solved the problem.