I think changing the quotes didn't help because you were using Microsoft Word or some other desktop publisher to edit your code. I took the first two lines of your source and replaced the quotes. I ended up with this source (named xml.php) which gave the correct error (below):
Code:
<?php 
header("Content-type: text/xml"); 
echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>";
?>
Code:
XML Parsing Error: no element found
Location: http://localhost/lib/PHP/xml.php
Line Number 1, Column 39:

<?xml version="1.0" encoding="UTF-8"?>
--------------------------------------^