Cookie handling

[meowmeow]

my forum website is, for example, 'forum.foobar.com' and I want vb3 to read and accept users (from the cookies) who have already logged in to 'mail.foobar.com'
so users can go from mail, to forum, to download without having to login all the time.

can anyone point out where I should start? what i should do?

looking at login.php ln

PHP Code:

if ($bbuserinfo = $DB_site->query_first("SELECT ....... 
WHERE username = '" . addslashes(htmlspecialchars_uni($username)) . "'")) 


is $username taken from ln 141 (is that where the user writed his username and password at login?)

if so, where is it that vb3 checks the cookies to see if the user can login??