I recently found the need to run multiple CodeIgniter applications inside of a single codebase for development & test purposes. So, I rewrote a few lines of the primary index file (/index.php) to automatically route URLs like subdomain.domain.com to the CodeIgniter application within /system/application/subdomain.

As a catch-all, the snippet will route the "www" subdomain (and requests with no subdomain) to application/live.

This is a nice & tidy way to give your development, test and live versions of a site their own unique subdomains. Works great when passing the URLs to clients for testing & approval.