With the advent of Twitter, shortened URLs have become all the rage: http://bit.ly/5dk33b, http://imgur.com/9G2rL, http://www.reddit.com/9dfo1. This kind of thing really isn’t all that hard to do.

$short_code = substr(preg_replace(
  array('/\//', '/\+/'),
  array('-', '_'),
  base64_encode(sha1(serialize($data, $salt), true))
), 0, 7);

Your $salt might be something as simple as a manually generated random string. If your data is time sensitive, it could be microtime(). Fancier two-way conversions are doable, in which you convert your primary key into a base-62 number, but I’ve yet to find a need for that.