Answer by aularon for simple rss feed

You should output real <?xml version='1.0' encoding='ISO-8859-1'?>, not an entitized version. Since it's problematic due to php's own <?, echo it after the header:

<?php
//header('Content-Type: application/xml');
header('Content-Type: application/rss+xml; charset=utf-8');
echo '<?xml version="1.0" encoding="utf-8"?>';
?>
<rss version='2.0'>
<channel>
<title>feed title</title>
<description>this is my example</description>
<link>http://localhost:8888/redline</link>
<copyright>Copyright (C) 2010 sarmenhb</copyright>
<item>
<title>Example 1</title>
<description>This is the description of the first example.</description>
<link>http://www.example.com/example1.html</link>
<pubDate>Mon, 29 Dec 2008 22:10:00 -0600</pubDate>
</item>
<item>
<title>Example 1</title>
<description>This is the description of the first example.</description>
<link>http://www.example.com/example1.html</link>
<pubDate>Mon, 29 Dec 2008 22:10:00 -0600</pubDate>
</item>
<item>
<title>Example 1</title>
<description>This is the description of the first example.</description>
<link>http://www.example.com/example1.html</link>
<pubDate>Mon, 29 Dec 2008 22:10:00 -0600</pubDate>
</item>
<item>
<title>Example 1</title>
<description>This is the description of the first example.</description>
<link>http://www.example.com/example1.html</link>
<pubDate>Mon, 29 Dec 2008 22:10:00 -0600</pubDate>
</item>
<item>
<title>Example 1</title>
<description>This is the description of the first example.</description>
<link>http://www.example.com/example1.html</link>
<pubDate>Mon, 29 Dec 2008 22:10:00 -0600</pubDate>
</item>    
</channel>
</rss>