如何用标签包装所有文本节点？

Question

How to wrap all text nodes with tags?

Input:

<p>Before <span>text1 <i>text2</i> text3</span> after</p>

Desirable output:

<p><b>Before </b><span><b>text1 </b><i><b>text2</b></i><b> text3</b></span><b> after</b></p>

My code:

<?php

$html = <<<'HTML'
<div>
  <p>Before <span>text1 <i>text2</i> text3</span> after</p>
</div>
HTML;

$dom = new DOMDocument;
$dom->preserveWhiteSpace = false;
$html = mb_convert_encoding( $html, "HTML-ENTITIES", "UTF-8" );
@$dom->loadHTML( $html, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD );

$xpath = new DOMXPath( $dom );

$childrens = $xpath->query('//p[1]/descendant-or-self::node()');

foreach ( $childrens as $child ) {
  $child->ownerDocument->saveHTML( $child );
}

echo $dom->saveHTML();

Thanks!