How To Configure Forum In A Backend/ Frontend Configuration As Suggested By This Article ?

I am on the way to implement fontend /backend and wonder how to include a forum without using separate sub-domain for forum . just like this forum is accessible e.g http://www.yiiframework.com/forum/

I have following main directories :

htdocs/myapp/frontend

htdocs/myapp/backend

htdocs/myapp/common

htdocs/myapp/forum

My desire is to access forum from each sudomain e.g

example.com/forum

fontend.example.com/forum

backend.example.com/forum ( might not need for backend but for other sub domains we need it )

myname.example.com/forum

Is is possible to redirect frontend.example.com/forum to htdocs/myapp/forum with .htaccess ?

(htdocs/myapp/frontend/www/.htaccess obviously )

How you will do this ? I mean what is the better way , sub domain or sub-directory with .htaccess ( if possible)

(Note : forum is not visible e.g example.com/forum as htdocs/myapp directory is NOT Document root )