PHP SimpleXML:在特定位置插入节点

$xml = <<<XML<books>
    <book>
       <title>Book 1</title>
       <author>Author 1</author>
    </book>
    <book>
       <title>Book 2</title>
       <author>Author 2</author>
    </book>
</books>
XML;

$simpleXml = simplexml_load_string($xml);

$newBook = $simpleXml->addChild('book');
$newBook->addChild('title', 'Book 3');
$newBook->addChild('author', 'Author 3');

$newXml = $simpleXml->asXML();
echo $newXml;

    <book>
       <title>Book 1</title>
       <author>Author 1</author>
    </book>
    <book>
       <title>Book 2</title>
       <author>Author 2</author>
    </book>
    <book>
       <title>Book 3</title>
       <author>Author 3</author>
    </book>
</books>