I have a website where I want users that sign up to get their own subdomain. This subdomain is virtual, and every subdomain uses the same web server files.

I use PHP and Apache, and I know about Virtual Hosts, but I'm wondering where I need to put the vhosts code. First, I don't have access to httpd.conf. Second, I want this to be done automatically upon registration.

I've read about virtual hosts, but didn't find anything that answers my questions. Where do I add this code, and can I do it without having access to anything other than the webserver folder (not Apache?)