PHP Code:
<?php
header('Content-Type: text/xml'); // application/rss+xml also possible
echo '<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE rss PUBLIC "-//Netscape Communications//DTD RSS 0.91//EN"
"http://my.netscape.com/publish/formats/rss-0.91.dtd">';
//do something
?>