A free way to get currency conversion rates within in an api callback is api.fixer.io

https://api.fixer.io/latest?base=USD

Returns

{"base":"USD","date":"2017-11-20","rates":{"AUD":1.3235,"BGN":1.6601,"BRL":3.2585,"CAD":1.2788,"CHF":0.99109,"CNY":6.634,"CZK":21.703,"DKK":6.3164,"GBP":0.75494,"HKD":7.8122,"HRK":6.4212,"HUF":265.21,"IDR":13524.0,"ILS":3.5139,"INR":65.103,"JPY":112.13,"KRW":1095.9,"MXN":18.97,"MYR":4.1481,"NOK":8.2589,"NZD":1.4636,"PHP":50.727,"PLN":3.5915,"RON":3.9482,"RUB":59.542,"SEK":8.453,"SGD":1.3557,"THB":32.77,"TRY":3.9225,"ZAR":14.055,"EUR":0.84882}}

Now its about simply using a json decode inside a function that returns what we want:

function usdtoaud($usd) {
    $data = json_decode(file_get_contents('https://api.fixer.io/latest?base=USD'));
    $aud = $data->rates->AUD;
    $result = $usd * $aud;
    return $result;
}

If we go echo usdtoaud(10); it will return 13.235 meaning $10USD gets us $13.235 AUD. To reverse this and get AUD to USD:

function audtousd($aud) {
    $data = json_decode(file_get_contents('https://api.fixer.io/latest?base=AUD'));
    $usd = $data->rates->USD;
    $result = $aud * $usd;
    return $result;
}

You can also make conversions for all the available currencies in the api call, like EUR to USD or CAD to USD etc etc.