simplexml_load_file() not working when trying to load intranet RSS

I'm trying to load a wordpress RSS feed using simplexml_load_file and failing.

I have a PHP script that lives on our internal web server (intranet) and a wordpress blog that lives on intranet/newsletter. I'm trying to pull the RSS feed from the blog into our intranet using the following code ad failing:

$rss = simplexml_load_file('intranet/newsletter/feed/');

This is the error I get:

Warning: simplexml_load_file(): I/O warning : failed to load external entity "intranet/newsletter/feed/" in /var/www/html/intranet/test.php on line 69

I've tried using

$rss = simplexml_load_file('http://intranet/newsletter/feed/');

with no success.

From a browser intranet/newsletter/feed/ works fine.