Make External Board Software Submodule

I have a site that is made of Yii and I would like to integrate external Board Software Like PHPBB3 or MyBB. But I don’t want them to be so separated that I want them to be modules.

I want user to type something like and is taken to a forum

What way can I go to do that? What are cons?

Is there alternative that bring same result (site to act as one not two glued)



Edit: One of the reasons is I want to use some Yiish thing like extensions and widgets in MyBB forum.