It's been a while but I integrated PHPBB with a few sites I've created, it was a bit fiddly, but there were a few guides on the forums on how to integrate the login into your own php code. As has been mentioned before it's important to keep them up to date for security as it tends to constantly get probed by hackers. And I can attest to the spam you get as others have said, definitely make sure you've a decent captcha/ human check on there.

Thanks - opted out now :D