3. Currency provider

The CurrencyProvider allows you to get the available currencies, it implements the CurrencyProviderInterface.

$currencyRepository = new EntityRepository();
$currencyProvider = new CurrencyProvider($currencyRepository);
$availableCurrencies = $currencyProvider->getAvailableCurrencies();

foreach ($availableCurrencies as $currency) {
    echo $currency->getCode();
}