generating an unique string based on user id
<p>I need to convert user db id or username (or whatever else is unique for each account) to an unique 3-5 character string with characters and/or numbers.</p>

<p>Example:
id 12481 would look like HJ28.</p>

<p>It should work both ways, so when HJ28 is given I can convert it back to the number. What would be the easiest way of doing this? </p>
