$R = 6371; // km
        $dLat = deg2rad($lat2-$lat1);
        $dLon = deg2rad($lon2-$lon1);
        $a = sin($dLat/2) * sin($dLat/2) +
          cos(deg2rad($lat1)) * cos(deg2rad($lat2)) *
          sin($dLon/2) * sin($dLon/2);
 $c = 2 * atan2(sqrt($a),  sqrt(1-$a));
 $d = $R * $c;
 return $d;