code: public function distance($target, $y = null, $z = null)
    {
      if($target instanceof self)
      {
        $_x = $this->gps['x'] - $target->gps['x'];
        $_y = $this->gps['y'] - $target->gps['y'];
        $_z = $this->gps['z'] - $target->gps['z'];
      }
      else
      {
        $_x = $this->gps['x'] - $target;
        $_y = $this->gps['y'] - $y;
        $_z = $this->gps['z'] - $z;
      }
      return sqrt($_x*$_x + $_y*$_y + $_z*$_z);
    }
docs: Distance between two coordinates.
  
  @param Coordinate3D|int $target|$x the point to calculate distance to
  @param ?int $y the point to calculate distance to
  @param ?int $z the point to calculate distance to
  @return float distance