WordPress code to display Flickr album contents like a native WP gallery

new_gallery_shortcode_for_flickr.php

/* This is a handler that overwrites the WordPress [gallery] shortcode handling, showing the 
 * code that allows [gallery] to display images from public Flickr albums. Note all permissions 
 * on Flickr must be set to allow the album's images to be publicly visible and shared.
 *
 * In your WordPress content, you provide a shortcode parameter [gallery flickrid="12345"],
 * where 12345 is the ID of a Flickr album (from the gallery URL).
 *
 * You will need to fill in your Flickr API key where indicated in the $flickr_url variable near the beginning. 
 *
 * This code creates its own $attachments array for later use in the gallery display HTML. 
 *
 * Michael Kupietz
 * https://michaelkupietz.com
 * <wordpress@michaelkupietz.com>
 */
 
add_filter("post_gallery", "my_post_gallery", 10, 2); //attach my_post_gallery function to filter post_gallery 

function my_post_gallery($output, $attr)
{        
    extract(
        shortcode_atts(
            [
                
                "flickrid" => "",
                /* .
                   .
                   .
                   rest of shortcode parameters go here
                   .
                   .
                   .
                 */
            ],
            $attr
        )
    );

//flickr code
    if ($flickrid !== "") {
        $flickr_url = "https://www.flickr.com/services/rest/?method=flickr.photosets.getPhotos" .
            "&api_key=INSERT YOUR FLICKR API KEY HERE" . 
            "&photoset_id=" . urlencode($flickrid) .
            "&format=json&nojsoncallback=1";
        
        $response = wp_remote_get($flickr_url);
        
        if (!is_wp_error($response)) {
            $photos = json_decode(wp_remote_retrieve_body($response), true);
            
            if (isset($photos['photoset']['photo'])) {
                $_attachments = [];
                foreach ($photos['photoset']['photo'] as $photo) {
                    $attachment = new stdClass();
                    $attachment->ID = $photo['id'];
                    
                    // Create a fake wp_prepare_attachment_for_js response
                    $img = [
                        'id' => $photo['id'],
                        'title' => $photo['title'],
                        'filename' => $photo['title'],
                        'url' => "https://live.staticflickr.com/{$photo['server']}/{$photo['id']}_{$photo['secret']}_b.jpg",
                        'link' => "https://live.staticflickr.com/{$photo['server']}/{$photo['id']}_{$photo['secret']}_b.jpg",
                        'alt' => $photo['title'],
                        'sizes' => [
                            'thumbnail' => [
                                'url' => "https://live.staticflickr.com/{$photo['server']}/{$photo['id']}_{$photo['secret']}_q.jpg",
                                'width' => 150,
                                'height' => 150
                            ],
                            'medium' => [
                                'url' => "https://live.staticflickr.com/{$photo['server']}/{$photo['id']}_{$photo['secret']}_z.jpg",
                                'width' => 640,
                                'height' => 640
                            ],
                            'full' => [
                                'url' => "https://live.staticflickr.com/{$photo['server']}/{$photo['id']}_{$photo['secret']}_b.jpg",
                                'width' => 1024,
                                'height' => 1024
                            ]
                        ]
                    ];
                    
                    // Add the wp_prepare_attachment_for_js data directly
                    $attachment->prepared_js_data = $img;
                    
                    $_attachments[] = $attachment;
                }
                
                $attachments = [];
                foreach ($_attachments as $key => $val) {
                    $attachments[$val->ID] = $_attachments[$key];
                }
    
                // Override wp_prepare_attachment_for_js for these Flickr images
                add_filter('wp_prepare_attachment_for_js', function($response, $attachment) use ($attachments) {
                    if (isset($attachments[$attachment->ID]) && isset($attachments[$attachment->ID]->prepared_js_data)) {
                        return $attachments[$attachment->ID]->prepared_js_data;
                    }
                    return $response;
                }, 10, 2);
            }
        }
    }

    //end flickr code

    /* .
       .
       .
       rest of code to create HTML output of gallery goes here
       .
       .
       .
    */   
}