如何在Php中使用dom从html中提取递归列表？

$html = '<ul>
            <li>Item 1</li>
            <li>Item 2
                <ul>
                    <li>Subitem 1</li>
                    <li>Subitem 2</li>
                </ul>
            </li>
            <li>Item 3</li>
        </ul>';

$dom = new DOMDocument();
$dom->loadHTML($html);

$xpath = new DOMXPath($dom);
$items = $xpath->query("//ul/li");

foreach ($items as $item) {
    echo $item->nodeValue . "\n";
}

$html = '<ul>
            <li>Item 1</li>
            <li>Item 2
                <ul>
                    <li>Subitem 1</li>
                    <li>Subitem 2</li>
                </ul>
            </li>
            <li>Item 3</li>
        </ul>';

$dom = new DOMDocument();
$dom->loadHTML($html);

$xpath = new DOMXPath($dom);
$items = $xpath->query("//ul/li");

foreach ($items as $item) {
    echo $item->nodeValue . "\n";
}