code: static function getSymbol($unit)
      {
          switch ($unit) {
              case static::PIECE:
                  return 'pcs';
  
              // Length
              case static::METER:
                  return 'm';
              case static::CENTIMETER:
                  return 'cm';
              case static::MILLIMETER:
                  return 'mm';
              case static::INCH:
                  return 'in';
              case static::FOOT:
                  return 'ft';
  
              // Weight
              case static::KILOGRAM:
                  return 'kg';
              case static::GRAM:
                  return 'g';
  
              // Volume
              case static::CUBIC_METER:
                  return 'm³';
              case static::LITER:
                  return 'L';
              case static::MILLILITER:
                  return 'mL';
  
              // Duration
              case static::DAY:
                  return 'days';
              case static::HOUR:
                  return 'hours';
              case static::MINUTE:
                  return 'minutes';
              case static::SECOND:
                  return 's';
  
              default:
                  throw new InvalidArgumentException("Invalid unit '$unit'.");
          }
      }
docs: Returns the symbol for the given unit.
  
  @param string $unit
  
  @return int