How to efficiently find the closest locations nearby a given location

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I think what you’re trying to achieve could be done better using the Haversine formula in your SQL. Google has a tutorial on how to get the nearest locations in a MySQL database but the general idea is this SQL:

SELECT id, ( 3959 * acos( cos( radians(37) ) * cos( radians( lat ) )
  * cos( radians( lng ) - radians(-122) ) + sin( radians(37) ) 
  * sin( radians( lat ) ) ) ) AS distance
FROM markers
HAVING distance < 25
ORDER BY distance LIMIT 0 , 20;

Then all the work you need to do is done on the database, so you don’t have to pull all the businesses into your PHP script before you even check the distance.

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