Postcode/ZIP Code Radius

I'm currently in planning stages of a website idea and have a query regarding how to calculate whether a postcode is within a radius of another postcode (or what areas/postcodes are within a radius of a postcode).

I basically need a user to be able to enter a postcode/zip code and a radius value in say miles, then site work out which other postcodes in its database fall within that radius.

I plan to use PHP/Java for my website implementation.

Could someone advise me as to the best method for implementation of this? What options are there? Don't need code, just pointing towards some methods (free or paid).