I have a couple of files where I output xml based upon URI parameters passed into a PHP file.

So I end up getting urls like this:
menu_render.php?menu_id=4&format=xml

which outputs xml just fine.

The problem I'm having is that I'd like to be able to open this file and then transform it via a stylesheet like below. Unfortunately, it gives me the following error:
Warning: I/O warning : failed to load external entity "menu_render.php?menu_id=4&format=xml" in...

I thought I was able to so this before. Does anybody know what I'm doing wrong. Help!

Code:
$xsl=new DomDocument();
$xsl->load('xsl_arc.xsl');

$xslt=new XSLTProcessor();
$xslt->importStyleSheet($xsl);

$xml=new DomDocument();

//$menu=file_get_contents(urlencode('menu_render.php?menu_id=4&format=xml'));

$xml->load('menu_render.php?menu_id=4&format=xml');
//$xml->load('menu.xml');


$results=$xslt->transformToXml($xml);

echo $results;