如何用PHP编写TOC的东西？

$doc = new DOMDocument();
libxml_use_internal_errors(true);
$doc->loadHTML($html);
$xpath = new DOMXPath($doc);
$headers = $xpath->query("//h1|//h2|//h3");
$toc = "<ul>";
foreach ($headers as $header) {
    $toc .= "<li><a href='#" . $header->getAttribute("id") . "'>" . $header->nodeValue . "</a></li>";
}
$toc .= "</ul>";

preg_match_all("/<h([1-3])>(.*?)<\/h[1-3]>/", $html, $matches);
$toc = "<ul>";
foreach ($matches[0] as $i => $header) {
    $toc .= "<li><a href='#" . $matches[2][$i] . "'>" . $matches[2][$i] . "</a></li>";
}
$toc .= "</ul>";

$parsedown = new Parsedown();
$html = $parsedown->text($markdown);
$toc = "<ul>";
foreach ($parsedown->getHeaders() as $header) {
    $toc .= "<li><a href='#" . $header['id'] . "'>" . $header['text'] . "</a></li>";
}
$toc .= "</ul>";