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function add_responsive_class($content){

     $content = mb_convert_encoding($content, 'HTML-ENTITIES', "UTF-8");
     $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-responsive');
     }

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