Ok, you can get the value by doing this:
$currency = 'USD';
//get json response
$return = file_get_contents('http://data.mtgox.com/api/1/BTC'.$currency.'/ticker_fast');
//decode it (into an array rather than object [using 'true' parameter])
$info = json_decode($return, true);
//access the dollar value
$value = $info['return']['last_local']['value'];
It's a public API so you don't need to authenticate, you just get the contents of the page which is in JSON format and decode it into an array and access the value element.
You now know that 1 BTC = 90.20505 dollars - so therefore 1 dollar is equal to 0.0110858 Bitcoins. (1 / 90.20505).
You can do this with any currency.