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METHOD 1
This should roughly sort the items on distance in MySQL, and should work in SQLite.
If you need to sort them preciser, you could try using the Pythagorean theorem (a^2 + b^2 = c^2) to get the exact distance.
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SELECT *
FROM table
ORDER BY ((lat-$user_lat)*(lat-$user_lat)) + ((lng - $user_lng)*(lng - $user_lng)) ASC
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METHOD 2
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SELECT
id, (
6371 * acos (
cos ( radians($user_lat) )
* cos( radians( lat ) )
* cos( radians( lng ) - radians($user_lng) )
+ sin ( radians($user_lat) )
* sin( radians( lat ) )
)
) AS distance
FROM table
HAVING distance < 30
ORDER BY distance
LIMIT 0 , 20;