I upgraded the PHP version backing my WordPress setup to PHP7.3. Of course, something had to go wrong:

Warning: Use of undefined constant jq_syntax_htmlentities - assumed 'jq_syntax_htmlentities' (this will throw an Error in a future version of PHP) in ...... wp-content/plugins/jquery-syntax/jquery-syntax.php on line 37

There’s no updates for jquery-syntax plugin that I’m using. Let’s assume that the author will fix this if they ever release an update.

For the time being, opening jquery-syntax.php manually and editing jq_syntax_quote to replace passing what looks like a function object (is it in PHP? I can’t be bothered to check) with a string when invoking preg_replace_callback() does the trick:

function jq_syntax_quote($content) {
        $content = preg_replace_callback('/<(pre)(.*?)>(.*?)<\/pre>/imsu','jq_syntax_htmlentities', $content);
        $content = preg_replace_callback('/<(code)(.*?)>(.*?)<\/code>/imsu','jq_syntax_htmlentities', $content);

        return $content;
}

Note how jq_syntax_htmlentities received single-quotes around it.

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via blog.vucica.net