Hasin Hayder has a quick post about taking a URL in the opposite direction than most seem to go these days - from shortened to the long, full URL.

Now when you get the short url shortened by using any of these services, you dont know where your browser is taking you! so if you are interested to figure out the original url hiding behind these short url, you need to have a little knowledge on how these services actually work.

Then the short URL is hit, the HTTP response is in the 300 family and the browser is redirected to the correct location. He uses cURL in PHP to grab this header information and parse out the full-length URL to return both the URL requested (the shortened one) and the full-length it was generated to point to.