This plugin allows to use the password from a working phpBB installation
to log into a WordPress blog.
You still need to create users in the WordPress administration panel, but
the users may use their phpBB password to log in.
WordPress passwords are still valid to log in.
It’s important that the WordPress logins match the
of the Phpbb users. username_clean doesn’t contain any uppercase
characters, nor special ones. Check directly in the DB if in doubt.
Free Download: Download this wp-plugin