phpRiot.com: Shortening URLs for goo.gl with Google's URL Shortener API
In 2010 Google released its own URL shortener, which allows you to shorten URLs to use the goo.gl domain. In this article I will show you how to easily create your own short URLs using their new URL shortener API. Note: This API is still in labs, meaning the API is subject to change without notice.
The tutorial steps you through the process of getting an API key for your scripts to use for access and what the message needs to be for the POST request to the Google service. His example script defines the API key and endpoint as well as a basic "shortenUrl" method that takes in the long URL to shorten. The curl request posts the data as JSON and receives a json-encoded message back with the original URL and the ID you wanted it shortened to.