How to create subdomain automatically….

The easiest and most reliable way of doing this would be to use the XML API over port 2082 (port 2083 if using SSL).
The actual function for creating a subdomain in cPanel is an API1 function:

domain = subdomain to create

rootdomain = the domain under which to create the subdomain

dir = path beginning at the user’s home directory where pages for this subdomain will be stored

So if I wanted to create the subdomain something.example.com that points to ~/subdomains/something, I would have:

As for translating this all into PHP, PHP 5 class in the developer section of the cPanel Community Forums is

vailable for download to handle all of that for you.