DOMdocument query: if first appearance of keyword matches criteria, exit?

How would I modify the script below so that if the first appearance of the keyword in the content string is already enclosed in bold or strong, I escape the node replacement?

    $keyword = "test";

    $content = "this is a <strong>test</strong> phrase with the word "test" in it.
                in this example, nothing would be changed, since the first 
                appearance of the keyword is already in boldface";

    @$d = new DOMDocument();
    @$d->loadHTML($content);
    @$x = new DOMXpath($d);
    @$nodes = $x->query("//text()[contains(.,'$keyword') and not(ance开发者_JS百科stor::h1) and not(ancestor::h2) and not(ancestor::h3) and not(ancestor::h4) and not(ancestor::h5) and not(ancestor::h6) and not(ancestor::b) and not(ancestor::strong)]");
    if ($nodes && $nodes->length) {
        $node = $nodes->item(0);
        // Split just before the keyword
        $keynode = $node->splitText(strpos($node->textContent, $keyword));
        // Split after the keyword
        $node->nextSibling->splitText(strlen($keyword));
        // Replace keyword with <b>keyword</b>
        $replacement = $d->createElement('strong', $keynode->textContent);
        $keynode->parentNode->replaceChild($replacement, $keynode);
    }
    echo $d->saveHTML();

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In that specific case, use evaluate instead of query and change the XPath to count the elements that match the highlight criteria with

"count(//text()[contains(.,'$keyword') and (ancestor::b or ancestor::strong)])"

If that returns > 1 the keyword is already enclosed. You have to run this query before the other query.