Sharing PHP session data between domains

I had a few comments on my Secure PHP sessions post that I wrote almost a year ago. So, you run HTTP on one domain, and HTTPS on a different one, and need to share the information between them?

I must say, I have done almost no research on this. But if you at the same time as you change the session ID, you create an extra cookie with the new session id for the other domain aswell. I would assume that you will be able to pick up the session data on that domain too.