PHP/Mysql store location service with only country and zip code data

I have a database that contains the country and zip code of a list of stores. I have to build a simple page that allows a person to input the zip code and country, then a map will show the closest locations to the zip/country info they input.

For example if they enter USA/90210, the map would show all the stores closest to that location.

I was looking at the google maps API php/mysql example but it requires lat/long data.

Any ideas how to do this with just a database that contains zipcode and country?

Source: stackoverflow-php