Xml 新DOMDocument参数

PHP:5.2.4

$xml=new DOMDocument('1.0','utf-8');  
$root=$xml->createElement('root');   
$xml->appendChild($root);  
$book=$xml->createElement('book');  
$root->appendChild($book);  
$node=$xml->createTextNode('who move my apple');  
$book->appendChild($node);  
echo $xml->save('./xml.xml')?'ok':'error';  
Warning: domdocument::domdocument() expects parameter 2 to be long, string given in C:\wamp\www\xu\index.php on line 2


Fatal error: Call to undefined method domdocument::createElement() in C:\wamp\www\xu\index.php on line 3

为什么警告和参数会正确写入应该是什么