Actually it looks like it is possible. Check out the
phpMyWebHosting project's
documentation.
Another alternative would be to have a PHP script write virtual hosts out of the database to a new conf file for inclusion in the main Apache conf file.
The script would be easy to write, but the process would be scary to automate in my opinion because it would require PHP being able to write to Apache conf files and restart Apache! Scary...
However, add a little human intervention and just use PHP to dump you an up to date virtual hosts conf file, and it might be kind of slick.