public function distance($lat1=0, $lon1=0, $lat2=0,  $lon2=0)
    {
          $theta = $lon1 - $lon2;
          $dist = sin(_deg2rad($lat1)) * sin(_deg2rad($lat2)) + cos(_deg2rad($lat1)) * cos(_deg2rad($lat2)) * cos(_deg2rad($theta));
          $dist = acos($dist);
          $dist = _rad2deg($dist);
          $dist = $dist * 60 * 1.1515;
          $dist = $dist * 1.609344;

          return $dist;
    }