Since I seem to have a fav on geo-calculation since Google-Maps has hit me, I have been looking for some information how to calculate distances between to points on the globe.

The GoogleAPI actually has a method to calculate the distance between points based on a shperical model.

But what if you'd like to implement a radius search on the database or calculate distances on the backend. I found a nice introduction for the subject over at MeridianWorldData, which takes into account various levels of precision to gain performance, where it is needed.

Also I found a PHP based example for distance calculation as well as a series of articles on the subject of GIS mapping.