code: public function withAreaInSqKm(string $areaInSqKm = null): self
      {
          $new = clone $this;
          $new->areaInSqKm = null === $areaInSqKm ? null : (float) $areaInSqKm;
  
          return $new;
      }
docs: @param string|null $areaInSqKm
  
  @return CountryInfo