ZIP Code proximity - how to calculate?

[eluser]Mirage[/eluser]
[quote author="spheroid" date="1209688468"]Anyone know of any good PHP libraries to figure out ZIP code proximity?[/quote]

I wrote something like that for a client. Buy a Zip Code db that includes Latitude and Longintude for the various codes. I get mine at ZipCode Download. Then in what ever table you're tracking add columns for Latitude and Longitude and update the the table with values for the zipcode table. Doing it that way saves you a time consuming join when doing the query.

Then the secret bit of salt is a SQL query that can approximate all included latitude/longitude coordinates given your starting point. Check it out at Endless Summer - a site I wrote in CI.

Cheers!