我有这样一个函数：

function add_responsive_class($content)
{
    $content = mb_convert_encoding($content, \'HTML-ENTITIES\', "UTF-8");
    if (!empty($content)) {
        $document = new DOMDocument();
        libxml_use_internal_errors(true);
        $document->loadHTML(utf8_decode($content));

        $imgs = $document->getElementsByTagName(\'img\');
        foreach ($imgs as $img) {
            $img->setAttribute(\'class\', \'img-fluid\');
        }

        $html = $document->saveHTML();
        return $html;
    }
}
add_filter(\'the_content\', \'add_responsive_class\');

比如--><img class ="img-fluid align-left"