code: private function getUnitOfVariation(array $variation)
      {
          foreach (self::$units as $unit) {
              if ((int) $unit['id'] === (int) $variation['unit']['unitId']) {
                  return $unit['unitOfMeasurement'];
              }
          }
  
          return null;
      }
docs: @param array $variation
  
  @return null|string