URL Layout Question

Here's my basic situation. I have a place on a webpage where teachers can great their own webpages. Currently to get to one of the users pages you have to go here:

http://www.siteurl.com/webapps/teach...NAME/index.php

I would like to be able to do:

http://www.teachers.siteurl.com/USERNAME

I tried creating a subdomain for teachers and having it redirect to the teacherwebpage directory and just hoping that then by adding the username at the end it would pick up the index.php file but didn't seem to work.

Sorry if this is a simple question.

Thanks for the help.

Ed