Solution to http://stackoverflow.com/questions/3056990/transform-rss-feed-into-another-standard-xml-format-with-php/3057592#3057592

<?php error_reporting(-1);

$xml = <<< XML
<?xml version="1.0" encoding="UTF-8"?>
    <rss version="2.0">
        <channel>
            <title>Name des RSS Feed</title>
            <description>Feed Beschreibung</description>
            <language>de</language>
            <link>http://xml-rss.de</link>
            <lastBuildDate>Sat, 1 Jan 2000 00:00:00 GMT</lastBuildDate>
            <item>
                <title>Titel der Nachricht</title>
                <description>Die Nachricht an sich</description>
                <link>http://xml-rss.de/link-zur-nachricht.htm</link>
                <pubDate>Sat, 1. Jan 2000 00:00:00 GMT</pubDate>
                <guid>01012000-000000</guid>
            </item>
            <item>
                <title>Titel der Nachricht</title>
                <description>Die Nachricht an sich</description>
                <link>http://xml-rss.de/link-zur-nachricht.htm</link>
                <pubDate>Sat, 1. Jan 2000 00:00:00 GMT</pubDate>
                <guid>01012000-000000</guid>
            </item>
            <item>
                <title>Titel der Nachricht</title>
                <description>Die Nachricht an sich</description>
                <link>http://xml-rss.de/link-zur-nachricht.htm</link>
                <pubDate>Sat, 1. Jan 2000 00:00:00 GMT</pubDate>
                <guid>01012000-000000</guid>
            </item>
        </channel>
    </rss>
XML;

$xsl = <<< XSL
<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<items>
  <xsl:copy-of select="//item">
    <xsl:apply-templates/>
  </xsl:copy-of>
</items>
</xsl:template>
</xsl:stylesheet>
XSL;

   $xslDoc = new DOMDocument();
   $xslDoc->loadXML($xsl);

   $xmlDoc = new DOMDocument();
   $xmlDoc->loadXML($xml);

   $proc = new XSLTProcessor();
   $proc->importStylesheet($xslDoc);
   echo $proc->transformToXML($xmlDoc);

/** output

X-Powered-By: PHP/5.3.2
Set-Cookie: ZDEDebuggerPresent=php,phtml,php3; path=/
Set-Cookie: ZendDebuggerCookie=127.0.0.1%3A10137%3A0||084|77742D65|1000; path=/
Content-type: text/html

<?xml version="1.0"?>
<items><item>
                <title>Titel der Nachricht</title>
                <description>Die Nachricht an sich</description>
                <link>http://xml-rss.de/link-zur-nachricht.htm</link>
                <pubDate>Sat, 1. Jan 2000 00:00:00 GMT</pubDate>
                <guid>01012000-000000</guid>
            </item><item>
                <title>Titel der Nachricht</title>
                <description>Die Nachricht an sich</description>
                <link>http://xml-rss.de/link-zur-nachricht.htm</link>
                <pubDate>Sat, 1. Jan 2000 00:00:00 GMT</pubDate>
                <guid>01012000-000000</guid>
            </item><item>
                <title>Titel der Nachricht</title>
                <description>Die Nachricht an sich</description>
                <link>http://xml-rss.de/link-zur-nachricht.htm</link>
                <pubDate>Sat, 1. Jan 2000 00:00:00 GMT</pubDate>
                <guid>01012000-000000</guid>
            </item></items>




*/