We've encountered this requirement for http://openguides.org. Here, alongside the lat + long, we store planar X-Y coordinates in the database, using a projection such as UTM or Ordnance Survey. Then, we use Pythagoras in SQL queries to find all entries within the distance.
This works well for city guides - our main application, but less well over large distances, where curvature of the oblate spheroid that is the Earth, becomes a factor.
I don't know what your application is, but check out OpenGuides, as there's probably quite a bit of code you can reuse. Or even set up a Guide yourself, if that's what you are trying to achieve.
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