hoping somebody here can help me. 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.