Install CurrencyPHP.
composer require tomwright/currency-php
composer require tomwright/currency-php-fixerio-rate-fetcher
FixerIORateFetcher is just a RateFetcher for CurrencyPHP.
$rateFetcher = new FixerIORateFetcher();
$gbp = new Currency('GBP', $rateFetcher);
$usd = new Currency('USD', $rateFetcher);
$priceInGBP = 100;
$priceInUSD = $gbp->convertTo($usd, $priceInGBP);
echo $priceInUSD; // 126