Haversine formula for finding all the rows in your database which are within N miles from given coordinates. SQL and PHP implementation below. http://en.wikipedia.org/wiki/Haversine_formula

haversine.php

$lat = 0; // some latitude
$lng = 0; // some longitude
$dst = 100; // some distance in miles

// get the contents of the file in the gist below
$sql = file_get_contents('haversine.sql')

// prepare PDO query
$statement = $db->prepare($sql);

// bind parameters
$statement->bindValue(1, $lat, PDO::PARAM_STR);
$statement->bindValue(2, $lng, PDO::PARAM_STR);
$statement->bindValue(3, $lat, PDO::PARAM_STR);
$statement->bindValue(4, $lng, PDO::PARAM_STR);
$statement->bindValue(5, $lat, PDO::PARAM_STR);
$statement->bindValue(6, $dst, PDO::PARAM_INT);

// execute query
$db->execute();

haversine.sql

-- The ? parameters will be supplied at run time

SELECT  username, firstname, lastname, zip, lat, lng, town,
        ( 3959 *
                acos(
                        COS(RADIANS(?)) *
                        COS(RADIANS(?)) * 
                        COS(RADIANS(lat)) * 
                        COS(RADIANS(lng)) 
                        +
                        COS(RADIANS(?)) * 
                        SIN(RADIANS(?)) * 
                        COS(RADIANS(lat)) * 
                        SIN(RADIANS(lng)) 
                        + 
                        SIN(RADIANS(?)) * 
                        SIN(RADIANS(lat))
                    )
            ) AS distance 
FROM employees
HAVING distance <= ?
ORDER BY distance
LIMIT 0 , 20;

This formula you are using is buggy. Please check this -> http://stackoverflow.com/questions/42522005/select-points-from-map-database-according-to-radius