Setting cookie for two domains.

Hi, How can I register a session or cookie for two domains. I want to maintain logged user's state when he goes from one domain to other domain. Example Suppose a user is logged in on domain1.com. A cookie is set and user is authenticated from database on the basis of that cookie. When he goes from domain1.com to domain2.com, I want he remains loggedin on domain2.com. I do not want any query string. Is it possible by playing with cookie or session settings? Please advise if anybody has faced such problem. Regards http://www.phpgurru.com