Add rss xmlns namespace definition to a php simplexml document?

I'm trying to create an itunes-valid podcast feed using php5's simplexml:

<?php   
$xml_string = <<<XML
<?xml version="1.0" encoding="UTF-8"?>


<channel>
</channel>
XML;

$xml_generator = new SimpleXMLElement($xml_string);
           $tnsoundfile = $xml_generator->addChild('title', 'Main Title');
           $tnsoundfile->addChild('itunes:author', "Author", ' ');
           $tnsoundfile->addChild('category', 'Audio Podcasts'); 
           $tnsoundfile = $xml_generator->addChild('item');
           $tnsoundfile->addChild('title', 'The track title');        
           $enclosure = $tnsoundfile->addChild('enclosure');
           $enclosure->addAttribute('url', 'http://test.com');
           $enclosure->addAttribute('length', 'filelength');
           $enclosure->addAttribute('type', 'audio/mpeg');       
           $tnsoundfile->addChild('itunes:author', "Author", ' '); 


header("Content-Type: text/xml");
echo $xml_generator->as开发者_运维知识库XML();

?>

It doesn't validate, because I've got to put the line:

<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0">

as per http://www.apple.com/itunes/podcasts/specs.html.

So the output SHOULD be:

<?xml version="1.0" encoding="UTF-8"?>
<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0">
<channel>

etc. I've been over and over the manual and forums, just can't get it right. If I put, near the footer:

header("Content-Type: text/xml");
echo '<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0">';
echo $xml_generator->asXML();
?>

Then it sort of looks right in firefox and it doesn't complain about undefined namespaces anymore, but feedvalidator complains that

line 1, column 77: XML parsing error: :1:77: xml declaration not at start of external entity [help]

because the document now starts:

<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0"><?xml version="1.0" encoding="UTF-8"?>

and not

<?xml version="1.0" encoding="UTF-8"?><rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0">

The code in shown in the question doesn't work because it doesn't use the correct namespace. Specifically, these lines:

$tnsoundfile->addChild('itunes:author', "Author", ' ');

They will create an <author/> node in the " " (one space) namespace, which is obviously incorrect. It should read:

$tnsoundfile->addChild('itunes:author', "Author", 'http://www.itunes.com/dtds/podcast-1.0.dtd');

This is the correct way to use namespaces.

This is very much possible with SimpleXML. Just declare the namespace within the constructor string, not as an attribute.

$rss_xml = new SimpleXMLElement(
   '<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd"/>');
$rss_xml->addAttribute('version', '2.0');