PHP URL shortener script that can shorten over 4 billion URLs

* Can shorten over 4 billion unique URLs in 8 or less characters (4,294,967,295 URLs to be exact)
* Super duper fast—uses very little server resources
* Includes an API so you can create your own short URLs on the fly
* Option to turn clickthru tracking on and off
* Option to limit usage to 1 IP address for personal use and to prevent spamming from others
* Only uses alphanumeric characters so all browsers can interpret the URL
* Secure—several data filters in place to prevent SQL injection hacks
* Uses 301 redirects for SEO and analytics yumminess (thanks Nicolas)
(Link: PHP URL shortener script that can shorten over 4 billion URLs)