Now, I want users who are logged in at somedomain.com to be logged in at theotherdomain.com.
In older versions of WordPress, this can be done by merely defining USER_COOKIE and PASS_COOKIE in wp-config.php. However, now there is another cookie item which is PHPSESSID and values vary for user on somedomain.com and theotherdomain.com.
How can I, at least, make the PHPSESSID value the same for a single user accessing both different domain or would anyone tell me how it is constructed?
I don’t know about that poppacket. Isn’t that what all modern browsers are setup to defend against? Even if you did get it going all it would take is the slightest of errors to have one big-ass security disaster on your hands.
I could see a subdomain but what you are talking about could turn out to be quite nasty.