The easiest way to do this is to do a bounding box query in SQL. However, if you're using MySQL, there are also some really useful geospatial extensions that make queries of this nature much more accurate and straightforward.

I'm using one on this mobile application: http://www.petproductmarketing.co.uk/mobile/directory_entry.php?entry=28

I'm not near my server at the moment, so don't have access to the query (which is pretty complicated), but if you Google for geospatial MySQL you should find something you can use.