Cross domain login and datasharing

[coffee_bean]

I have two sites - a "main" site and a site for the forums, both on different locations and servers. The main site has been static until now, but I hope to convert it all to PHP for various reasons.

I am hoping to offer some resources that are available to registered users, and it seems silly to ignore the fact that I have a good user base on the forum site, and for consistency sharing the usernames and passwords seems a good idea.

My intention so far has been to allow users to log in on the site with their forum un/pw, and then to authenticate that and keep a session for them locally. There is no need to require them to be logged in on both at once, I just need to authenticate the user, and then keep their user ID for later use in the session.

I have remote access to the mySQL server, and have been trying as above, but don't seem to be able to get it to work. Here is the SQL query I have tried:

SELECT userid FROM user WHERE username='me' and password = MD5(CONCAT('mypassword', salt))

Any ideas? I have tried just MD5('password') and all sorts - shall I just be honest and say I don't know how its done!? That's why I'm here...