MyBB Community Forums

I am trying to re-organize my site so that, for example:

http://www.domain.com/aboutus.php

turns into this:

http://www.domain.com/aboutus/

and if I manually type this in my web browser:

http://www.domain.com/aboutus/index.php, it will return the same file as above.

How would I go about approaching this?

I appreciate any tips on this, thanks.

EDIT:

Another specific example, say I wanted to change this:

http://community.mybb.com/portal.php

into this:

http://community.mybb.com/portal/

which will be the same as this:

http://community.mybb.com/portal/index.php

Hope that is more clear.