Determining Nearest Location by Zip Code

Hi~ I was wondering if anyone has played with or developed a script that allows someone to input a zip code. It will then perform a query that checks a specified radius (say 50 miles) and return results of all the entries in your MySQL db that fall within that radius based on zip code. So, let's say you had a list of shops across the U.S. You want someone to be able to enter their zip and it will tell them the shops closest to them or within a specified radius.... Anyone have ideas on how to do this? Thanks, -Ben