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.
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