PHPhacks can be somewhat dubious, and not a good place to go for scripts in general. People are too quick to throw that name out there. The thing is that they will publish any and all hacks known to man, and some of those do not take proper security precautions or just plain poorly written.
With that said ... here is a
blank page template from phpBB.com to get you started. Here is a
snippet of code to add to that to check user authentication. The only thing you need to worry about is setting the $phpbb_root_path variable correctly in order to use the functions of phpbb.
Most hacks deal with the phpBB system and not with external systems. It is good that you do not have another User authentication set up, phpBB's can be used.
What I suggest is looking into "portal" solutions for phpBB. There are a couple published at phpBB.com already which should give you the tools necessary to effect the changes.
Another mod for phpBB is called the
phpBB FetchAll Mod (listed in SourceForge) -- this is probably the better choice that I could recommend, it will give you a library of functions to interact with the phpBB system itself.
You wouldn't know it by looking at it, but
my site is totally run by phpBB in the backend (no there is no forum there ... hehehe). Even the Blogger blog is integrated into it. I will be using the User Authentication here in the future for a project I will be adding in hopefully not too long from now.
I am using some parts of phpBB FetchAll and some home-grown coding of my own.