Thread:Extension talk:Phpbb Single Sign-On/Phpbb 3.0.11 and mediawiki 1.20.2/reply (7)

Set your cookie domain in phpbb to just "domain.com" instead. That should allow phpbb sessions to cover your entire domain. (If it still doesn't, try adding the leading dot: ".domain.com")

You can also check your cookie path in the ACP and make sure it's just / and not /forums/, as you want the cookie to work for the whole domain; especially in the case of having your wiki outside your forum folder/in another subdomain.

This approach worked for me, and there's lots of other things about how to get your entire site integrated with phpbb here: phpbb topic: Full Site Integration I have it pointed specifically to a post about cookies not working outside the forums. I hope that thread helps some of the other issues involving phpbb's end! :3