It depends on how you are planning on integrating it. If you mean adding a gallery to a post made on a phpBB site, that probably won't work since it wouldn't parse the PHP code. If you want to edit the PHP source code of a phpBB page itself, then yeah that should work fine assuming PowerGallery and phpBB are on the same server. Check out the Add to Site tab in the admin for the PHP code chunk you'll have to copy into your phpBB page.
-justin