invoiceninja/app/Libraries/Currency/Conversion/CurrencyApi.php

<?php
/**
 * Invoice Ninja (https://invoiceninja.com)
 *
 * @link https://github.com/invoiceninja/invoiceninja source repository
 *
 * @copyright Copyright (c) 2020. Invoice Ninja LLC (https://invoiceninja.com)
 *
 * @license https://opensource.org/licenses/AAL
 */

namespace App\Libraries\Currency\Conversion;

use App\Models\Currency;
use Illuminate\Support\Carbon;

class CurrencyApi implements CurrencyConversionInterface
{

	public function convert($amount, $from_currency_id, $to_currency_id, $date = null)
	{

		if(!$date)
			$date = Carbon::now();

		$from_currency = Currency::find($from_currency_id);

		$to_currency = Currency::find($to_currency_id);

		$usd_amount = $this->convertToUsd($amount, $from_currency);

		return $this->convertFromUsdToCurrency($usd_amount, $to_currency);
		
	}

	public function exchangeRate($from_currency_id, $to_currency_id, $date = null)
	{

		$from_currency = Currency::find($from_currency_id);

		$to_currency = Currency::find($to_currency_id);

		$usd_amount = $this->convertToUsd(1, $from_currency);
		
		return $this->convertFromUsdToCurrency($usd_amount, $to_currency);

	}

	/**
	 * Converts a currency amount to USD
	 * 
	 * @param  float  $amount   amount
	 * @param  object $currency currency object
	 * @return float            USD Amount
	 */
	private function convertToUsd($amount, $currency)
	{
		return $amount / $currency->exchange_rate;
	}

	/**
	 * Converts USD to any other denomination
	 * 
	 * @param  float  $amount   amount
	 * @param  object $currency destination currency
	 * @return float            the converted amount
	 */
	private function convertFromUsdToCurrency($amount, $currency)
	{
		return $amount * $currency->exchange_rate;
	}

}