I've noticed some hosting providers have Apache setup so that if I create a directory (foo.example.com) Apache will automatically know that the DNS entry for foo.example.com routes to that document root.
What I am to do is setup something like this:
<VirtualHost *:80> ServerName *.example.com DocumentRoot /home/user/*.example.com </VirtualHost>
Where the DocumentRoot would match based on the pattern from the ServerName wild card.
In doing so all I should need to do is create
/home/user/foo.example.com after the configuration is in place and not need to modify anything in Apache. This would allow me to add sub-domains on the fly without needing to restart or reload or even edit anything in Apache.
A use case would be something like this.
Where I wouldn't need to do anything but simply make the directory.