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Editorial Team
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Asked: 2026-06-01T01:40:25+00:00

I’ve installed a syntax highlighter, but in order for it to work, the tags

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I’ve installed a syntax highlighter, but in order for it to work, the tags must be written as &lt; and &gt;. What I need to do is replace all <‘s with &lt; and >’s with &gt; but only inside the PRE tag.

So, in short, I want to escape all HTML characters inside of the pre tag.

Thanks in advance.

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1 Answer

  1. Editorial Team

    tl;dr

    You need to parse the input HTML. Use the DOMDocument class to represent your document, parse the input, find all <pre> tags (using findElementsByTagName) and escape their content.

    Code

    Unfortunately, the DOM model is very low-level and forces you to iterate the child nodes of the <pre> tag yourself, to escape them. This looks as follows:

    function escapeRecursively($node) {
    if ($node instanceof DOMText)
        return $node->textContent;

    $children = $node->childNodes;
    $content = "<$node->nodeName>";
    for ($i = 0; $i < $children->length; $i += 1) {
        $child = $children->item($i);
        $content .= escapeRecursively($child);
    }

    return "$content</$node->nodeName>";
}

    Now this function can be used to escape every <pre> node in the document:

    function escapePreformattedCode($html) {
    $doc = new DOMDocument();
    $doc->loadHTML($html);

    $pres = $doc->getElementsByTagName('pre');
    for ($i = 0; $i < $pres->length; $i += 1) {
        $node = $pres->item($i);

        $children = $node->childNodes;
        $content = '';
        for ($j = 0; $j < $children->length; $j += 1) {
            $child = $children->item($j);
            $content .= escapeRecursively($child);
        }
        $node->nodeValue = htmlspecialchars($content);
    }

    return $doc->saveHTML();
}

    Test

    $string = '<h1>Test</h1> <pre>Some <em>interesting</em> text</pre>';
echo escapePreformattedCode($string);

    Yields:

    <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
<html><body><h1>Test</h1> <pre>Some &lt;em&gt;interesting&lt;/em&gt; text</pre></body></html>

    Note that a DOM always represents a complete document. Hence when the DOM parser gets a document fragment it fills in the missing information. This makes the output potentially different from the input.

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