implement geo / spatial / proximity search with php and mysql --- what have you done successfully For my website's users, I want to implement a worldwide geo / spatial / proximity search with php and mysql.

I want to do this as simply and minimally as possible.

What is the minimum data I should capture (Country, State, City, Postal Code, etc.)?

There are dozens out-of-the-box and open-source solutions out there, with varying reviews. What have you tried that worked or did not work?

It's quite easy to implement such a thing for the U.S. (e.g., Google has some Zip code related proximity-solutions), but apparently if I want to go global, it's an order of magnitude more difficult. Have you tried (and was it worth-it) to code it from scratch?
I think using Latitude and Longitude is the best bet. Because when you go global, then zipcode might not be very accurate search. There is a very good article about using spatial indexes and the Point type database fields. You can read here
http://vinsol.com/blog/2011/08/30/geoproximity-search-with-mysql/