ok, so it appears to matter not whether i use the 'www.' prefix. Either way i can generate the following cookies (at least)
wD_Sess_User-5147 (where 5147 is my userID on webdip see:http://webdiplomacy.net/profile.php?
userID=5147 )
phpbb3_6q7fq_u
phpbb3_6q7fq_sid
phpbb3_6q7fq_k
The following phpbb3 cookies, which i believe are auto-generated when you open the new forum. (though they seem to be missing the www. when i'm using that link, which is odd, i presume is part of some debugging you've been trying.)
(along with some google analytics cookies which i will ignore).
i'm pretty sure you want to generate PHPbb cookies when a user logs in, and persist them (which i know can be annoying...) and would force all users to log out and back in again once - to access the new forums.
At the moment i can't use the (new) forum page (see here: http://www.webdiplomacy.net/contrib/phpBB3/ucp.php?mode=login )
Which i presume would work, except it isn't detecting usernames from webdip.
I also assume you'd rather the links redirect to the regular webdip login page (see: http://webdiplomacy.net/logon.php ) so that all users are using a single page for login, rather than feeling like they were logging into the (new) forum and game site separately.
I used to work on a webgame where we had separate forums hosted on the same site, and it was annoying sorting out cookies and session ids, we never managed to integrate properly, so i've got feels for the task you're trying.