No results when using a web API

I'm attempting to pull an XML page from the U.S. Postal Service (USPS) rate calculator, using PHP. Here is the code I am using (with my API login and password replaced of course):

<?
$api = "http://production.shippingapis.com/ShippingAPI.dll?API=RateV4&XML=<RateV4Request ".
       "USERID=\"MYUSERID\" PASSWORD=\"MYPASSWORD\"><Revision/><Package ID=\"1ST\">".
       "<Service>FIRST CLASS</Service><FirstClassMailType>PARCEL</FirstClassMailType>".
       "<ZipOrigination>12345</ZipOrigination><ZipDestination>54321</ZipDestination>".
       "<Pounds>0</Pounds><Ounces>9</Ounces><Container/><Size>REGULAR</Size></Package></RateV4Request>";

$xml_string = file_get_contents($api); 

$xml = simplexml_load_string($xml_string);
?>

Pretty straightforward. However it never returns anything. I can paste the URL directly into my browser's address bar:

http://production.shippingapis.com/ShippingAPI.dll?API=RateV4&XML=<RateV4RequestUSERID="MYUSERID" PASSWORD="MYPASSWORD"><Revision/><Package ID="1ST"><Service>FIRST CLASS</Service><FirstClassMailType>PARCEL</FirstClassMailType><ZipOrigination>12345</ZipOrigination><ZipDestination>54321</ZipDestination><Pounds>0</Pounds><Ounces>9</Ounces><Container/><Size>REGULAR</Size></Package></RateV4Request>

And it returns the XML I need, so I know the URL is valid. But I cannot seem to capture it using PHP. Any help would be tremendously appreciated. Thanks in advance.

One thing is that you need to URL encode the XML that you are sending to the service. The browser will do that for you automatically, but file_get_contents won't.

Try this:

 $param = urlencode("<RateV4Request ".
   "USERID=\"MYUSERID\" PASSWORD=\"MYPASSWORD\"><Revision/><Package ID=\"1ST\">".
   "<Service>FIRST CLASS</Service><FirstClassMailType>PARCEL</FirstClassMailType>".
   "<ZipOrigination>12345</ZipOrigination><ZipDestination>54321</ZipDestination>".
   "<Pounds>0</Pounds><Ounces>9</Ounces><Container/><Size>REGULAR</Size></Package></RateV4Request>");

 $api = "http://production.shippingapis.com/ShippingAPI.dll?API=RateV4&XML="
        .$param;

 ... then the rest of the code

If that doesn't help, make sure you have error reporting activated so you get a response if file_get_contents has an error.