Add Custom class image (in the_content attached images) - Wordpress

custom_class_image.php

<?php

// Input:
// <img src="...">
// Output:
// <a href="../wp-content/uploads/2016/09/2016-012_0005.jpg" data-lightbox="roadtrip">
//    <img src=".../wp-content/uploads/2016/09/2016-012_0005-150x150.jpg" alt="2016-012_0005" width="150" height="150" class="custom-class alignleft size-thumbnail wp-image-1048">
// </a>


function custom_class_image($content){
        $content = preg_replace('/<img (.*) \/>\s*/iU', '<a href="" data-lightbox="roadtrip"><img \1 /></a>', $content);
        $content = mb_convert_encoding($content, 'HTML-ENTITIES', "UTF-8");
        $document = new DOMDocument();
        libxml_use_internal_errors(true);
        $document->loadHTML(utf8_decode($content));

        foreach ($document->getElementsByTagName('img') as $k => $img) {
            $src = $img->getAttribute('src');
            // Add custom class.    
            // $existing_class = $img->getAttribute('class');
            // $img->setAttribute('class', 'custom-class '.$existing_class);

            $args = array(
                 'post_type' => 'attachment',
                 'numberposts' => -1,
                 'post_status' => null,
                 'post_parent' => get_the_ID(),
             );

            // Get the attached images.
            $attachments = get_posts( $args );
            $img->parentNode->setAttribute('href', $attachments[$k]->guid);
        }

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

add_filter('the_content', 'add_responsive_class');