Can't get my PHP-generated RSS to serve properly

Hey all -

Glad some of you found that sample data helpful.   :-)

I use PHP/MySQL to generate RSS feeds of my podcasts. The feed is submitted as *.xml and I use .htaccess to redirect it to my PHP document. The start of the document sets the right header and outputs the <? & ?> to prevent PHP from trying to process the leading XML line as code (this is cleaned up a bit for readability):

<?php
header("content-type: application/rss+xml");
echo '<?';
?>
xml version="1.0" encoding="UTF-8"
<?php
echo '?>';
?>
<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" xmlns:atom="http://www.w3.org/2005/Atom" version="2.0">

This has always worked fine on one podcast, but on a new one it's not. You can see the results here:
http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Finfactvideo.com%2Fpodcast.php

It's throwing a 500 error, a parsing error, and complaining that feeds should not be served with the "text/html" type, even though I'm serving the right header. Other PHP pages on this site work fine, and there are no special Apache directives on my site that works that are missing here. Can anyone suggest what I might be missing?

- Brian