Create a subdomain on account creation

I've like to be able to automatically create a subdomain of an account's primary domain when the account is created. The name of the subdomain will not change between accounts; it'll always be subdomain.<domain.tld>. I've found the documentation for both WHM hooks and the PHP API very confusing, so any pointers would be great.

I'd prefer to use PHP to do all this, since I'm unfamiliar with Perl. That said, if it's incredibly simpler to do in Perl, I'd be open to that route as well.