i know that internet explorer does really funny things for me when i try to view xml documents. your browser might be interpreting the xml as html and therefore your page might appear blank. OR, your browser might be trying to interpret the xml as a mime type or something....all those xml elements would look like unknown html tags.
try sending mime type info with header().
OR use htmlspecialchars() on the xml before you ECHO it to the browser. for instance, if your xml is stored in a variable $my_xml then do this
echo htmlspecialchars($my_xml);
hth