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->asXML();   

    ?>

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: <unknown>:1:77: xml declaration not at start of external entity

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">

Any ideas gratefully received! Thank you.

    Well, the conclusion is that php simplexml isn't capable of producing an itunes-store-compliant podcast as specced at:
    http://www.apple.com/itunes/podcasts/specs.html

    So, here's the workaround:

    $xml_result = $xml_generator->asXML();
$xml_LeftHandSide = '<?xml version="1.0" encoding="UTF-8"?><rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0">';
$xml_RightHandSide = substr($xml_result, 22);
echo $xml_LeftHandSide . $xml_RightHandSide . "</rss>";

    et voila! feedvalidator and itunes are happy 🙂

    Wish I'd thought of this 5 nights ago...

      2 years later

      I had a similar problem two years later, but I found a much simpler solution. By simply adding the XML-namespaces to the XML-Code:

      $xml = new SimpleXMLElement('<Array xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" />');
$xml->addChild('Content', 'The Content');
echo $xml->asXML();

      The output was:

      <Array xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" >
    <Content>The Content</Content>
</Array>

      In your Code these lines would have been right:

      $xml_string = <<<XML
    <?xml version="1.0" encoding="UTF-8"?>
    [B]<rss xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0"> [/B]
    <channel>
    </channel>
    [B]</rss>[/B]
    XML;

      Important: don't forget the closing </rss>-tag at the end!
      Alternatively you could add <channel></channel> with

      $channel = $xml_generator->addChild('channel');
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