code: protected function asCurrencyStringFallback($value, $currency = null)
      {
          if ($currency === null) {
              if ($this->currencyCode === null) {
                  throw new InvalidConfigException('The default currency code for the formatter is not defined.');
              }
              $currency = $this->currencyCode;
          }
  
          return $currency . ' ' . $this->asDecimalStringFallback($value, 2);
      }
docs: Fallback for formatting value as a currency number.
  
  @param string|int|float $value the value to be formatted.
  @param string $currency the 3-letter ISO 4217 currency code indicating the currency to use.
  If null, [[currencyCode]] will be used.
  @return string the formatted result.
  @throws InvalidConfigException if no currency is given and [[currencyCode]] is not defined.
  @since 2.0.16