PHP authentication with multiple domains and subdomains

Question

I have one main domain: main.com, subdomains: test1.main.com, test2.main.com and other domains one.com, two.com

Answer 1

But if user reach domains one.com directly, than one.com can't know if user had login by the right answer mentioned above, seems like must use some extra js, it's jsonP! we let account.main.com/userLoginStat.php?callback=loginThisDomain to check if user had login main.com, if so, the js callback function loginThisDomain do some thing to autologin user to one.com.