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Editorial Team
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Asked: 2026-05-17T21:37:47+00:00

I need to highlight a keyword in a paragraph, as google does in its

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I need to highlight a keyword in a paragraph, as google does in its search results. Let’s assume that I have a MySQL db with blog posts. When a user searches for a certain keyword I wish to return the posts which contain those keywords, but to show only parts of the posts (the paragraph which contain the searched keyword) and to highlight those keywords.

My plan is this:

  • find the post id which has the searched keyword in it’s content;
  • read the content of that post again and put each word in a fixed buffer array (50 words) until I find the keyword.

Can you help me with some logic, or at least to tell my if my logic is ok? I’m in a PHP learning stage.

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1 Answer

  1. Editorial Team

    If it contains html (note that this is a pretty robust solution):

    $string = '<p>foo<b>bar</b></p>';
$keyword = 'foo';
$dom = new DomDocument();
$dom->loadHtml($string);
$xpath = new DomXpath($dom);
$elements = $xpath->query('//*[contains(.,"'.$keyword.'")]');
foreach ($elements as $element) {
    foreach ($element->childNodes as $child) {
        if (!$child instanceof DomText) continue;
        $fragment = $dom->createDocumentFragment();
        $text = $child->textContent;
        $stubs = array();
        while (($pos = stripos($text, $keyword)) !== false) {
            $fragment->appendChild(new DomText(substr($text, 0, $pos)));
            $word = substr($text, $pos, strlen($keyword));
            $highlight = $dom->createElement('span');
            $highlight->appendChild(new DomText($word));
            $highlight->setAttribute('class', 'highlight');
            $fragment->appendChild($highlight);
            $text = substr($text, $pos + strlen($keyword));
        }
        if (!empty($text)) $fragment->appendChild(new DomText($text));
        $element->replaceChild($fragment, $child);
    }
}
$string = $dom->saveXml($dom->getElementsByTagName('body')->item(0)->firstChild);

    Results in:

    <p><span class="highlight">foo</span><b>bar</b></p>

    And with:

    $string = '<body><p>foobarbaz<b>bar</b></p></body>';
$keyword = 'bar';

    You get (broken onto multiple lines for readability):

    <p>foo
    <span class="highlight">bar</span>
    baz
    <b>
        <span class="highlight">bar</span>
    </b>
</p>

    Beware of non-dom solutions (like regex or str_replace) since highlighting something like “div” has a tendency of completely destroying your HTML… This will only ever “highlight” strings in the body, never inside of a tag…

    Edit Since you want Google style results, here’s one way of doing it:

    function getKeywordStubs($string, array $keywords, $maxStubSize = 10) {
    $dom = new DomDocument();
    $dom->loadHtml($string);
    $xpath = new DomXpath($dom);
    $results = array();
    $maxStubHalf = ceil($maxStubSize / 2);
    foreach ($keywords as $keyword) {
        $elements = $xpath->query('//*[contains(.,"'.$keyword.'")]');
        $replace = '<span class="highlight">'.$keyword.'</span>';
        foreach ($elements as $element) {
            $stub = $element->textContent;
            $regex = '#^.*?((\w*\W*){'.
                 $maxStubHalf.'})('.
                 preg_quote($keyword, '#').
                 ')((\w*\W*){'.
                 $maxStubHalf.'}).*?$#ims';
            preg_match($regex, $stub, $match);
            var_dump($regex, $match);
            $stub = preg_replace($regex, '\\1\\3\\4', $stub);
            $stub = str_ireplace($keyword, $replace, $stub);
            $results[] = $stub;
        }
    }
    $results = array_unique($results);
    return $results;
}

    Ok, so what that does is return an array of matches with $maxStubSize words around it (namely up to half that number before, and half after)…

    So, given a string:

    <p>a whole 
    <b>bunch of</b> text 
    <a>here for</a> 
    us to foo bar baz replace out from this string
    <b>bar</b>
</p>

    Calling getKeywordStubs($string, array('bar', 'bunch')) will result in:

    array(4) {
  [0]=>
  string(75) "here for us to foo <span class="highlight">bar</span> baz replace out from "
  [3]=>
  string(34) "<span class="highlight">bar</span>"
  [4]=>
  string(62) "a whole <span class="highlight">bunch</span> of text here for "
  [7]=>
  string(39) "<span class="highlight">bunch</span> of"
}

    So, then you could build your result blurb by sorting the list by strlen and then picking the two longest matches… (assuming php 5.3+):

    usort($results, function($str1, $str2) { 
    return strlen($str2) - strlen($str1);
});
$description = implode('...', array_slice($results, 0, 2));

    Which results in:

    here for us to foo <span class="highlight">bar</span> baz replace out...a whole <span class="highlight">bunch</span> of text here for

    I hope that helps… (I do feel this is a bit… bloated… I’m sure there are better ways to do this, but here’s one way)…

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