Update 19 February 2017 – FYI, bbPress is a WordPress plugin that can add a forum to your WordPress site. It’s not nearly as sophisticated and functional as phpBB but it may be good enough for your needs.
It’s not too surprising that people will want to integrate WordPress and phpBB. WordPress after all is an extremely popular blogging program, but really it’s a lightweight content management system. I use it for this site. Why? Because it’s much easier than doing everything by hand with HTML or even a fancy web page program like Dreamweaver. There are thousands of themes and tens of thousands of plugins that let you extend the functionality of WordPress in lots of ways. The quality of some of these plugin are amazing. The design of some of WordPress themes are as well. All this plus it is free, except possibly for some plugins where you get nagged to buy a “professional” version.
In my mind, WordPress is the best-of-breed lightweight content management system out there and a phenomenal blogging tool. But WordPress has limits. Like any content management system, at some point it becomes not quite enough. Try managing a hundred pages with WordPress, for example. WordPress also has some drawbacks. Unlike phpBB, where a modification and extension review is rigorous and must be approved by a peer-reviewed team of developers to get listed, in WordPress it’s pretty straightforward to get a plugin listed if you follow their coding standards. The quality of plugins are thus suspect.
There are forum plugins for WordPress that are generally decent. None come close to having the quality and breadth of phpBB. This is not surprising. phpBB is a best-of-breed forum solution. It is continuously updated, quality reviewed and has a passionate set of developers who strive to write very good code. Unsurprising I get regular requests to integrate phpBB into WordPress.
WordPress and phpBB can coexist, as long as you keep each in their own spaces. For example you could put the forum in a /forum folder and have the rest of the site in WordPress. However, phpBB can’t use WordPress’s authentication system, and visa versa. phpBB styles don’t match WordPress themes. A number of clever WordPress developers have tried to bridge the two platforms. The solutions are inelegant. For example, WP-United tries to create a common authentication system, so that if you are a forum user you will also be a user in WordPress. Unfortunately, as of this writing, WP-United supports phpBB 3.0, not 3.1. It’s really just one developer and he seems to not be actively supporting it. Worse, when I have installed it I found it flaky so I didn’t warrant the work. BridgeDD is another effort, but it too only supports phpBB 3.0.
The bigger problem is that clients naturally want phpBB to work inside their WordPress theme, but it won’t. That’s not to say I haven’t done this for a number of clients. I can usually make it work but takes considerable trial and error. Sometimes there are conflicts between WordPress and phpBB styles, such as using similar names for classes. The result is often a solution that looks nice but may run slowly. The larger problem is that if the WordPress theme changes, phpBB won’t automatically pick up the new style changes. Someone has to retrofit it, and that requires someone with expertise in HTML, CSS, phpBB and WordPress.
So phpBB and WordPress don’t dance together and probably never will. One approach I sometimes recommend is to put the forum into a subdomain, like forum.mysite.com so the user sort of expects it won’t look the same. You can also create a domain just for the forum, like mysiteforum.com and link the two. This allows both phpBB and WordPress to inhabit separate areas and do their things elegantly while also setting the user expectation that both have separate purposes.
If you do want to integrate them expect a non-optimal experience at best.
Feel free to contact me if you want me to try but do expect the work to be costly, generally in the hundreds of dollars range, if it can be done at all.