PHP searches nearby based on gps coordinates

define('EARTH_RADIUS', 6370.6935);

function vicinity($lng, $lat, $distance = 0.5) {
    //$distance = 0.5;      // 单位 10KM
    $radius = EARTH_RADIUS;

    $dlng = rad2deg(2*asin(sin($distance/(2*$radius))/cos($lat)));
    $dlat = rad2deg($distance*10/$radius);

    $lng_left = round($lng - $dlng, 6);
    $lng_right = round($lng + $dlng, 6);
    $lat_top = round($lat + $dlat, 6);
    $lat_bottom = round($lat - $dlat, 6);

    return array('lng'=> array('left'=> $lng_left, 'right'=> $lng_right), 'lat'=> array('top'=> $lat_top, 'bottom'=> $lat_bottom));
}

PHP, gps