SimpleXMLElement creating two XML Tags

I'm building xml with the SimpleXMLElement Object from PHP.

While doing so I encountered the following problem, which i can't solve:

I'm generating the root xml element like this:

$xml = new SimpleXMLElement("<?xml version=\"1.0\" encoding=\"utf-8\" ?>");

But i get a 2 XML headers when i do echo $xml->asXML(); like this:

<?xml version="1.0"?>
<xml version="1.0" encoding="UTF-8"></xml>

Which is obvioulsy wrong. But how can i fix this so i only get the

<xml version="1.0" encoding="UTF-8">

part?