Fixed lenght random code generator

I need to assign a random name to a PDF file of 20 digits (numbers). This code has to be generated from a unique person ID.

Example:

Person ID                                           File name
----------                                          ------------------------
40352142N ---------> random code generator -------> XXXXXXXXXXXXXXXXXXXX.pdf

It would be great if you could generate the person ID from the file name as well. It must be reversible but only if you know the formula.

What would be the best way to do it in PHP or ASP. Any ideas on how to elegantly implement this?

Thank you!