framework/i18n/Formatter.php
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);
    }
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);
    }
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