Currency conversion API on a shoestring

Someone just came to our table at Mashed08 and asked if Yahoo! offers a currency conversion API. We don’t, but a few lines of PHP allows you to get the information from the Yahoo finance site:

function convert($from,$to){
 $url= 'http://finance.yahoo.com/currency/convert?amt=1&from='.$from.'&to='.$to.'&submit=Convert';
 $ch = curl_init();
 curl_setopt($ch, CURLOPT_URL, $url);
 curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
 $feed = curl_exec($ch);
 curl_close($ch);
 preg_match_all("/tabledata1\">([^<]+)<\/td>/",$feed,$cells);
 return $cells[1][1];
}
echo convert(’USD’,'GBP’);

There’s a whole list of currency codes available on oanda.

A few more lines turns this into a JSON API:

header('Content-type:text/javascript');
$from = $_GET['from'];
$to = $_GET['to'];
$callback = $_GET['callback'];
if(preg_match("/[A-Z|a-z]{3}/",$to) && preg_match("/[A-Z|a-z]{3}/",$from)){
  $to = strToUpper($to);
  $from = strToUpper($from);
  $url= ‘http://finance.yahoo.com/currency/convert?’ .  
        ‘amt=1&from=’.$from.’&to=’.$to.’&submit=Convert’;
  $ch = curl_init();
  curl_setopt($ch, CURLOPT_URL, $url);
  curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
  $feed = curl_exec($ch);
  curl_close($ch);
  preg_match_all("/tabledata1\">([^<]+)<\/td>/",$feed,$cells);
  if(is_numeric($cells[1][1])){
    $out = ‘{"from":"’.$from.’","to":"’.$to.’","factor":"’.$cells[1][1].’"}’;
  } else {
    $out = ‘{"error":"Could not convert currencies, are you sure about the names?"}’;
  }
} else {
  $out = ‘{"error":"Invalid Currency format, must be three letters"}’;
}
if(isset($callback)){
  if(preg_match("/[a-z|A-Z|_|-|\$|0-9|\.]/",$callback)){
    $out = $callback.’(’.$out.’)';
  } else {
    $out = ‘{"error":"Invalid callback method name"}’;
  }
}
echo $out;

You have several parameters:

• from (mandatory): three letter currency code (upper or lower case)
• to (mandatory): three letter currency code (upper or lower case)
• callback (optional): the name of the callback method that should be wrapped around the resulting object

If something goes wrong, the API will return an object with an error property, otherwise you’ll get an object with three properties:

• from: the original currency
• to: the target currency
• factor: the conversion factor

Say you store this as convert.php somewhere, then you could do the following:

<script type="text/javascript"> 
  function converted(obj){
    if(obj.error){
      alert(obj.error);
    } else {
      alert('one ' + obj.from + ' is ' + obj.factor + ' ' + obj.to);
    }
  }
</script>
<script type="text/javascript" src="convert.php?from=gbp&to=usd&callback=converted"></script>

This is a terrible dirty hack and if Yahoo finance ever changes their HTML (and they will), this will cease to work.

Tags: api, conversion, currency, finance, hacking, lackofapi, mashed08

This entry was posted on Saturday, June 21st, 2008 at 7:27 pm and is filed under hackday, hacking, hacks. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.