how to generate xml dynamically with sibling

how can i add the sibling when i generate the code with for loop

<root>
   <Book>
      <type serial_no="1" name="little monkey"/>
      <type serial_no="2" name="little monkey"/>
   </Book>
   <CD>
      <type serial_no="1" name="ABC song"/>
      <type serial_no="2" name="ABC song"/>
   </CD>
</root>

My code is as follow

$xml = new SimpleXMLElement('<root/>');

for ($i = 1; $i <= 2; ++$i) {

$stock = $xml->addChild('Book');
$stock ->addChild('Type', 'serial_no="'.$i.'name="little monkey"');
$stock = $xml->addChild('CD');   
$stock ->addChild('Type', 'serial_no="'.$i.'name="ABC song"');
}

Header('Content-type: text/xml');
print($xml->asXML());

I know there are something wrong with my code Please give me some suggestion