php simplexml 如何删除节点

<?php
//[需求]：删除所有书籍的year节点
$sxe = simplexml_load_file("bookstore.xml");
foreach ($sxe->book as $book) {
unset($book->year); //删除year节点。删除变量用unset()。
}
 
$sxe->asXML('sxe_delete_book.xml');

<?xml version="1.0" encoding="utf-8" ?>
<bookstore>
 <book category="COOKING">
 <title>Everyday Italian</title>
 <author>Giada De Laurentiis</author>
 <year>2005</year>
 <price>30.00</price>
 </book>
 <book category="CHILDREN">
 <title>Harry Potter</title>
 <author>J K. Rowling</author>
 <year>2005</year>
 <price>29.99</price>
 </book>
 <book category="WEB">
 <title>Learning XML</title>
 <author>Erik T. Ray</author>
 <year>2003</year>
 <price>39.95</price>
 </book>
</bookstore>