Google provides a simple and free* API for querying currency exchange rate. You can convert the currency using google apis url which are given below

https://www.google.com/finance/converter?a=$amount&from=$from_Currency&to=$to_Currency

Where a is the amount .We can do this by two methods which are given below.
1 By using file_get_contents

   function currency($fromCur,$toCur,$amount) {
                $amount = urlencode($amount);
                $fromCur = urlencode($fromCur);
                $toCur= urlencode($toCur);
                $get = file_get_contents("https://www.google.com/finance/converter?a=$amount&from=$fromCur&to=$toCur");
                $get = explode("<span class=bld>",$get);
                $get = explode("</span>",$get[1]);  
                $convertedamount = preg_replace("/[^0-9\.]/", null, $get[0]);
                 return round($convertedamount,2000);

                 }

         echo currency("USD","INR",2000);

2 By using the curl method

   function currency($fromCurrency, $toCurrency, $amount) {
        $url = "https://www.google.com/finance/converter?a=" . $amount . "&from=" . $fromCurrency . "&to=" . $toCurrency;

        $ch = curl_init();
        $timeout = 0;
        curl_setopt ($ch, CURLOPT_URL, $url);
        curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt ($ch, CURLOPT_CONNECTTIMEOUT, $timeout);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        $rawdata = curl_exec($ch);
        curl_close($ch);

        $matches = array();
        preg_match_all("|<span class=bld>(.*)</span>|U", $rawdata, $matches);

        $result = explode(" ", $matches[1][0]);

        return round($result[0], 2);
    }

    $usd = currency("USD", "ETB", 2000);
    echo "2000 USD = " . $usd . " ETB";

By these ways we will get the raw data which requires regular expression for the conversion result.