Class RatesDB {

    private $rates = [];

    public function __construct($rates) {
        //I think here should be the function that get's rates from fixerio
    }

    public function getRate($currency)
    {
        return $rates[$currency];
    }

}

Class Currency {

    public $code;
    public $rate;

    public function __construct(string $code, RatesDB $rates) {

        $this->code = $code;

        $this->rate = $rates->getRate($code);

    }

}

Class Money {

    public $amount;
    public $currency;

    public function __construct(integer $amount, Currency $currency) {

        $this->amount = $amount;

        $this->currency = $currency;

    }

    public function getConvertedEquivelant(Currency $convertedCurrency): Money
    {

        $baseValue = $this->amount / $this->currency->rate;

        return new Money(($baseValue * $convertedCurrency->rate), $convertedCurrency);

    }

}