Class FixerConverter implements Converter {

    private $baseCurrency;

    private $convertedCurrencies = [];

    public function __construct(string $baseCurrency)
    {
        $this->baseCurrency = $baseCurrency;
    }

    private function getConversionRate(string $currency): string
    {
        if (!isset($savedConversionRates[$currency])) {

            $conversionRateJson = @file_get_contents('http://api.fixer.io/latest?base=' . $this->baseCurrency . '&symbols=' . $currency));

            if ($conversionRateJson === false) {
                echo 'Error: unexpected error retrieving currency conversion rate from API service.';
            }

            $conversionRateArr = json_decode($conversionRateJson, TRUE);

            $conversionRate = $conversionRateArr['rates'][$currency];

            $this->savedConversionRates[$currency] = $conversionRate;

            return $conversionRate;

        } else {
            return $savedConversionRates[$currency];
        }
    }

    public function convertToCurrency(string $amount, string $currency): float
    {
        return $amount * $this->getConversionRate($currency);
    }
}