All subdomains routed to same DocumentRoot, apache config

Question

You've seen it done on sites like basecamphq.com where you would have username.basecamphq.com

I'm wanting to do a similar thing, any subdomain is routed through to the same index.php file.

so username1.example.com will request /home/some/path/to/www (in here is index.php) and username2.example.com will request the same file

I have it working locally, but my server uses cPanel. What would be your guess at the apache conifguration?