Troubles with importing a node from another DOMDocument

Hi!

I have a trouble with importing a node from one DOMDocument to another. I
have no troubles with MSXML lib, but during porting my sourcecode to PHP i
faced this:

<?
$xml = new DOMDocument('1.0', 'utf-8');
$xml2 = new DOMDocument('1.0', 'utf-8');

$el = $xml->createElement("smth");
$el2 = clone $el;

$xml2->appendChild($el2);
?>

OUTPUT:

Fatal error: Uncaught exception 'DOMException' with message 'Wrong Document
Error' in D:\www\joomla\libraries\bc\2.php:9
Stack trace:
#0 D:\www\joomla\libraries\bc\2.php(9):
DOMNode->appendChild(Object(DOMElement))
#1 {main}
  thrown in D:\www\joomla\libraries\bc\2.php on line 9

I wonder, how can i solve this problem?