Do you need these domains to be run by other people, or do you just want your system to do different things if people come in on a different subdomain?
For the second:
You can register your site as *.mysite.com then have your webserver put all request to the same web root. In that tree you have the first thing any of your PHP scripts do be parsing the URL to find out what the full site name is. Once you have that then you can have your PHP script do whatever it needs to do to make the site look and feel different. Or even just send the user to a different set of scripts that do what you want that site to do.
We used this scheme at a startup I worked for where we'd build the look and feel of our site to match one of our clients, and then when somone came to our site from theres the URL would be clientssite.oursite.com and our site would look like theirs but function the same as ours.
