Live currency exchange script, to retrieve the current exchange rates betwee to currencies, live from the internet.

exchange.php

<?php

/**
 * Live currency exchange script.
 * This script allows you to retrieve the current currency exchanges, live, from the internet.
 *
 * This script is developed by Tim Visee, for educational purposes.
 * The Yahoo currency exchange API is used to retrieve the live exchange rates.
 *
 * ---
 *
 * Script usage:
 * To retrieve the current currency exchange rates for two currencies, simply make a request to this script with the
 * parameters that are described bellow.
 *
 * Script parameters:
 * - from: Three letter code for base currency.
 * - to: Three letter code for target currency.
 *
 * For example, to get the exchange rate for EUR to USD, use the following parameters:
 * index.php?from=EUR&to=USD
 * The currency exchange rate is returned as a numerical value, with a dot as decimal delimiter.
 *
 * The string 'ERROR' is returned if an error occurred. For example, if the current exchange rate couldn't be retrieved,
 * or if the three letter currency code is invalid.
 *
 * ---
 *
 * @author Tim Visee
 * @website http://timvisee.com/
 * @copyright Copyright (c) Tim Visee 2015. All rights reserved.
 */

// Disable all PHP errors
error_reporting(0);
ini_set('display_errors', 0);

/**
 * Show an error, and stop the script.
 */
function showError() {
    die('ERROR');
}

// Make sure the currencies are set
if(!isset($_GET['from']) || !isset($_GET['from']))
    showError();

// Get the currencies to convert
$from = trim(strtoupper($_GET['from']));
$to = trim(strtoupper($_GET['to']));

// Make sure both currency identifiers are three characters long
if(strlen($from) != 3 || strlen($to) != 3)
    showError();

// Create the conversion string
$conversionString = $from . $to;

// Do the whole exchange rate retrieval process in a try-catch block to handle possible errors
try {
    // Get the current exchange
    $url = "http://query.yahooapis.com/v1/public/yql?env=store://datatables.org/alltableswithkeys&q=SELECT%20*%20FROM%20yahoo.finance.xchange%20WHERE%20pair%20IN%20%28%22" . $conversionString . "%22%29";
    $ch = curl_init();
    curl_setopt($ch,CURLOPT_URL, $url);
    curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
    curl_setopt($ch,CURLOPT_CONNECTTIMEOUT, 4);
    $xml = curl_exec($ch);

    // Parse the XML data
    $xmlData = new SimpleXMLElement($xml);

    // Make sure the rate is set
    if(!isset($xmlData->results->rate->Rate))
        showError();

    // Get and parse the exchange rate as a float
    $exchangeRate = floatval($xmlData->results->rate->Rate);

    // Echo the actual exchange rate
    echo $exchangeRate;

} catch(Exception $e) {
    // Show an error page
    showError();
}