The only way you can croot PHP is to have it start as root, carry out the chroot and demote itsself to the user you want to run it as. There are programs that do this for you, such as suexec. To do this you need to have PHP run as a CGI not a web server module.

Another method you can use is the powerful URL rewriting module in Apache. A request to http://www.example.com/user/ gets rewritten to http://user.example.com/ (you can then have a name server running on the server which resolves unknown .example.com addresses to the IP address of the server).