So we have two goals. First, requests for the new URL are internally rewritten to call the existing scripts without users ever knowing they exist. Second, requests for the old URLs get a 301 redirect to the new URLs so that search engines and good bookmarks immediately switch to the new URLs.

He starts with a sample .htaccess file, showing a simple RewriteRule to take in the request and remap them back to the old PHP script's input format. They work through a few more changes, noting issues along the way (in case you hit them too) and end up with a simple, and much easier way to achieve clean URL bliss.