構造を壊さずに HTML で検索結果を強調表示するにはどうすればよいですか?

$doc = new DOMDocument;
$doc->loadXML($str);
$xp = new DOMXPath($doc);

// Search elements containing the search text
$r = $xp->query('//*[contains(., "'.$search.'")]/*[FALSE = contains(., "'.$search.'")]/..', $anchor);

// Process search results
foreach($r as $i => $node)
{
    // Extract search text nodes and create suitable nodes if necessary
    $range = new TextRange($xp->query('.//child::text()', $node));
    $ranges = array();
    while(FALSE !== $start = strpos($range, $search))
    {
        $base = $range->split($start);
        $range = $base->split(strlen($search));
        $ranges[] = $base;
    };

    // Wrap matching text nodes
    foreach($ranges as $range)
    {
        foreach($range->getNodes() as $node)
        {
            $span = $doc->createElement('span');
            $span->setAttribute('class', 'search_hightlight');
            $node = $node->parentNode->replaceChild($span, $node);
            $span->appendChild($node);
        }
    }
}