Code Sample: PHP Javascript Currency converter + exchange rates caching RSS feed 28

Problem: For a recent project I needed to get the latest exchange rates to allow the sales folks to quickly calculate costs  while doing estimates in different currencies. There are many sites online where you can do this, but we wanted to create an in-app widget that could be re-used on various internal  pages.

The first tasks was to find a  daily exchange rate feed preferably rss rather than screen scrape some sites, for our purposes the RSS (XML) feed from the ECB site proved adequate , because its updated daily and it comes from a reputable source.

Source Code on Gihub: https://github.com/acbrandao/Javascript

Exchange_Rates.php Class

The other feature of this was to cache the feed daily (3600 x 24 ) so we’re not hitting the ECB’s servers continuously making good use network bandwidth as well as snappy in-page performance.

<?php
/**
* @version $Id: exchange.php 4408 2010-12-03 19:26:30Z tonyb $
$Date: 2010-12-03 14:26:30 -0500 (Fri, 03 Dec 2012) $
$Revision: 1001 $
$Author: tonyb $
$HeadURL: $
$Id: exchange.php 4408 2010-12-03 19:26:30Z tonyb $
Functions to access Internet exchange rates from sources defaults to the ECB (European Central Bank) etc.
http://www.ecb.int/stats/eurofxref/eurofxref-daily.xml
Retrieves values from RSS feed of
*/

class exchange_rates
{

public $cache_folder=null;
protected $cache_file="cached_rates.xml"; //file holds chached rates
public $exchange_source_url=null;
public $exchange_rate_time=null;
private $exchrate=null; //array that holds exchange rates
public $cache_time= null; //time to hold the rates 12 hours

// Currency names that we'll use later on
public $names = array (
'USD' => "US Dollar",
'JPY' => "Japanese Yen",
'GBP' => "Pound Sterling",
'CAD' => "Canadian Dollar",
'HKD' => "Hong Kong Dollar",
'CNY' => "Chinese yuan renminbi",
'INR' => "Indian Rupee",
'AUD' => "Australian Dollar",
'SGD' => "Singapore Dollar",
'EUR' => "European Euro"
); //end of array

///////////////////////////////////////////////////
// constructor loads up the the rates URL
// makes the initial call to fetch the rates
public function __construct($url="http://www.ecb.int/stats/eurofxref/eurofxref-daily.xml")
{
//setup the default values
$this->exchange_source_url=$url;
$this->cache_folder=dirname(__FILE__); //default to current folder - in production place $_CONFIG['cache_path'] here.
$this->cache_time=(3600 * 24) ; //how long in seconds to make use of the cached file

$this->fetch_exchange_rates(); //now make the initial call to get the rates
}

/* Perform the mathematical conversion from -> to currency , returns converted currency */
public function exchange_rate_convert($from,$to,$amount)
{

if ($to=="EUR") //converting to EUR then find the Inverse of the currency
{
//echo "<font color=red>invert</font>";
if ( $this->exchrate[$to] == 0 || $this->exchrate[$from] == 0 )
{
echo "Error: Unable to retrieve exchange rates";
$value=0;
}
else
$value= $amount * (1 / $this->exchrate[$from] )/ $this->exchrate[$to];
}
else
{
if ( $this->exchrate[$from] == 0 )
$value=0;
else
$value= $amount * $this->exchrate[$to] / $this->exchrate[$from];
}
return $value;
}

/* Return the currency facotr*/
public function exchange_factor($currency)
{

return exchange_rate_convert("USD",$currency,1);
}

# Exchange rates - Grab current rates from European Central Bank all rates relative to the EUR currency
# 1 EUR equievelant to xxx currency
public function fetch_exchange_rates()
{
$cache_time;

$now = time();
$cache =$this->cache_folder."/".$this->cache_file; //location and filename of rates chached file
$this->exchrate['EUR'] = 1.00;
$amount = 1;
$interval=0;

# Check whether we have a recent copy of the data locally

if (file_exists($cache) )
$interval = $now - filemtime($cache);
if ( ( $interval > $cache_time ) || !file_exists($cache) ) //not in cahce OR cache expired
{
$stuff = file( $this->exchange_source_url);
$trace = "Fresh XML GET from URL";

if (is_writable($cache))
{
$fh = fopen ($cache,"w+");
foreach ($stuff as $line) {
fputs($fh,$line);
}
}
else
die("File $cache is not WRITABLE - check folder permissions");
}
else //in cache use that data
{
$stuff = file($cache);
$trace = "Using Cached data $interval seconds old";
}

// Extract data from file- conversion rates between each currency and the Euro
// Now lets loop through the feed and pull out the exhcnage rates
foreach ($stuff as $line)
{
if( preg_match("/time='([[:graph:]]+)'/",$line,$gotval) ) //found the time, save it..
{
$this->exchange_rate_time =$gotval[1]; //extract the value
};
preg_match("/currency='([[:alpha:]]+)'/",$line,$got_currency); //regex out the currency
if (preg_match("/rate='([[:graph:]]+)'/",$line,$got_rate)) //regex out the rate
{
$this->exchrate[$got_currency[1]] = $got_rate[1]; //assign it to the global array
}
} //end of for

} //end of function

///////////////////////////////////////////////////
// Just does a array dump of the exchange rates array
///////////////////////////////
public function show_rates()
{

echo "<pre>";
print_r($this->exchrate);
echo "</pre>";

}

} //end of class

?>

The JavaScript Interface (widget_currency_calc.php)

Finally the last part was to create simple JavaScript in-line calculations that allowed currency conversions to happen automatically in any direction when the user typed in the currency.

This was coded with a basic table and a series of event handlers to automatically trigger the currency updates. It was kept very simple (making no use of ajax or jquery) to keep the size and dependencies to a minimum.  The flag icons can be placed inside an images folder. Below is an example of one row of that widget code, download the assets file attached to see the full details.

Calling the Script

<?php
require "widget_currency_calc.php"; //import class containing the exchange rates
echo "<hr>";

$rates = new exchange_rates(); //instantiate the class

//Some Test rates
echo "1 USD equals ".$rates->exchange_rate_convert("USD","EUR",1) . " EUR <br>";
echo "1 USD equals ".$rates->exchange_rate_convert("USD","GBP",1) . " GBP <br>";
echo "1 USD equals ".$rates->exchange_rate_convert("USD","CAN",1) . " CAN <br>";
echo "1 USD equals ".$rates->exchange_rate_convert("USD","CNY",1) . " EUR <br>";
$rates->show_rates();

?>

DEMO : The full widget looks like this

Demo Rate Calculator (click to run)

 Download

The Complete files and assets here: currency_exchange

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