Añadir clase responsive a cada imagen del contenido WordPress

add_class_responsive.php

function add_responsive_class_content($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),LIBXML_HTML_NOIMPLIED);

        $imgs = $document->getElementsByTagName('img');
        foreach ($imgs as $img) {           
           $existing_class = $img->getAttribute('class');
           $img->setAttribute('class', "img-responsive $existing_class");
        }

        $html = $document->saveHTML();
        $html = str_replace(”,”,$html);
        return $html;
        }

        else {

        return $content;  
        }
       

}

add_filter('the_content', 'add_responsive_class_content');