Parse in currency rates of major cities
I see some inconsistencies in your getCurrency.php code. There are many variables that are never declared within the functions, so I don't think that's going to work as expected.

I think the best approach would be to make a parser which looks for whatever currency code you need, regardless of the item order because that might change in the future. I mean, the most performance-expensive part of the job is to parse the whole XML into a SimpleXML structure which you're doing anyway, so looping through the items seaching for the right one wouldn't be much worse and your code will be more straight-forward and escalable.

On the other hand, you might also want to have some cache of the parsed XML to avoid re-parsing the same file on each request. But that depends on the variability of the XML feed. Maybe you can check with The Money Converter how often the feed is updated.