Automated subdomain creation script

We are seeking development of a script (can be in PHP or Linux shell called through PHP) that will take an alphanumeric name from a variable in PHP and make this into a subdomain (subdomain set up in /var/www/html/users/) without requiring Apache to reboot. The idea is that through our website, a user signs up, defines their subdomain and then having paid, the subdomain is created through the script to be created. The new user gets a success message and can click further. We currently use an Ensim Pro Control Panel to manually create these subdomains. As we use a third-party managed dedicated server, please indicate what files you will need access to / have de-restricted.