php生成xml文件

<?php
$books = array();
$books[] = array(
    'title' => 'PHP Hacks',
    'author' => 'Jack Herrington',
    'publisher' => "O'Reilly"
);
$books[] = array(
    'title' => 'Podcasting Hacks',
    'author' => 'Jack Herrington',
    'publisher' => "O'Reilly"
);
$doc = new DOMDocument();
$doc->formatOutput = true;
$r = $doc->createElement("books");
$doc->appendChild($r);
foreach ($books as $book) {
    $b = $doc->createElement("book");
    $author = $doc->createElement("author");
    $author->appendChild($doc->createTextNode($book['author']));
    $b->appendChild($author);
    $title = $doc->createElement("title");
    $title->appendChild($doc->createTextNode($book['title']));
    $b->appendChild($title);
    $publisher = $doc->createElement("publisher");
    $publisher->appendChild($doc->createTextNode($book['publisher']));
    $b->appendChild($publisher);
    $r->appendChild($b);
}
echo $doc->saveXML();
?>