PHP计算当前坐标3公里内4个角落的最大最小经纬度

//$lng 、$lat 经纬度    
$half = 6371;
    $distance = 3; //3公里 
    $dlng = 2 * asin(sin($distance / (2 * $half)) / cos(deg2rad($lat)));
    $dlng = rad2deg($dlng);
    $dlat = $distance / $half;
    $dlat = rad2deg($dlat);
    $fourpoint = array(
      'left-' => array('lat' => $lat + $dlat, 'lng' => $lng - $dlng),
      'right-' => array('lat' => $lat + $dlat, 'lng' => $lng + $dlng),
      'left-bottom' => array('lat' => $lat - $dlat, 'lng' => $lng - $dlng),
      'right-bottom' => array('lat' => $lat - $dlat, 'lng' => $lng + $dlng)
);