This is possible to do in many few ways. How I would do it (I'm not suggesting it's the best way, it's just the way I think is the path of least resistance) is going to be to setup a page on your WordPress site and use a custom page template. That template can include everything you want to include, and then you can use curl, file_get_contents, or another method to grab it over http. I would recommend making this template output XML and wrapping your elements appropriately. For instance,
When you download the XML file, you'll want to cache it so that every phpBB page hit doesn't also require a hit to your WP site. In your phpBB code, you would parse the XML and use the parts as needed.
I would do it this way for three reasons:
- By loading the file over your domain, you'll always retrieve the correct content from the correct blog and not have to worry about setting the global blog_id, etc.
- This affords you the ability to scale horizontally and put phpBB on one server and WP on another someday.
- Your system will be more orthogonal, which makes it easier to maintain and test, as well as being generally more agile.
I hope this is clear enough; I can provide more examples if needed.