From the PHPSnippets.info site there's a recent post about using a conversion tool from Google to convert currency values from one type to another.
Just a few people know that Google have a built-in currencies calculator. (Just type 100$ in euros in Google search box if you want to test it) This calculator works very well, so what about using PHP and cURL and get this functionnality in your own web app?
The script makes a cURL request over to the Google API's calculator feature and passes in the amount to convert and the codes for the "to" and "from" (like USA, CAD or EUR). You'll need cURL support in your PHP installation to use the script, but it wouldn't be all that difficult to modify it to use a basic socket connection too.