EDIT

<?php

/**

 * Class GalleryItemMedia filters suitable MediaRepresentations from a CDM Media

 * object and prepares them for to be used as item in a MediaGallery.

 */

class GalleryItemMedia {

  public static $OK = 0;

  public static $ACCESS_DENIED = -1;

  public static $ERROR = -2;

  public static $image_mime_type_list_default =  [

    'image/jpg',

    'image/jpeg',

    'image/png',

    'image/gif',

  ];

  private $image_mime_type_list;

  private $thumbnail_width;

  private $thumbnail_height;

  private $thumbnail_representation = null;

  private $full_size_representation = null;

  private $web_app_representation = null;

  private $first_representation = null;

  /**

   * GalleryItemMedia constructor.

   */

  public function __construct($media, $thumbnail_width, $thumbnail_height, $image_mime_type_list = null) {

    if(!$image_mime_type_list){

      $this->image_mime_type_list  =  self::$image_mime_type_list_default;

    }

    $this->thumbnail_width = $thumbnail_width;

    $this->thumbnail_height =$thumbnail_height;

    $this->findRepresentations($media);

  }

  private function findRepresentations($media) {

    if (isset($media->representations[0]->parts[0])) {

      $thumbnail_representations = cdm_preferred_media_representations($media, $this->image_mime_type_list,

        $this->thumbnail_width, $this->thumbnail_height, FALSE, TRUE

      );

      $full_size_representations = cdm_preferred_media_representations($media, $this->image_mime_type_list);

      $web_app_representations = cdm_preferred_media_representations($media, $this->image_mime_type_list, NULL, NULL, TRUE);

      if (isset_not_empty($media->representations)) {

        // can be used as last resort fall back

        $this->first_representation = $media->representations[0];

      }

      if (isset_not_empty($this->thumbnail_representation)) {

        $this->thumbnail_representation = array_shift($thumbnail_representations);

      }

      if (isset_not_empty($full_size_representations)) {

        $this->full_size_representation = array_shift($full_size_representations);

      }

      if (isset_not_empty($web_app_representations)) {

        $this->web_app_representation = array_shift($web_app_representations);

      }

    }

  }

  public function getThumbnailImageMediaItem() {

    $url = $this->getThumbnailImageUrl();

    if($url){

      return new ImageMediaItem($this->getThumbnailImageUrl(), $this->getThumbnailSize());

    }

    return null;

  }

  /**

   * Provides the size of the thumbnail image if available.

   * In case this GalleryItemMedia contains as web app url the favicon

   * default size is returned.

   *

   * @return object|null

   */

  private function getThumbnailSize() {

    $repr = $this->getThumbnailRepresentation();

    if($repr){

      return $this->sizeOf($repr);

    } else  if($this->hasWebAppItem()) {

      // returning the favicon size, actual size is not important, in this case!

      return new MediaSize(64,64);

    }

    return null;

  }

  /**

   * The url of the thumbnail image if this is available. In case this

   * GalleryItemMedia contains as web app url the according favicon is returned

   * as thumbnail url.

   *

   * @return string|null

   *   The URL

   */

  private function getThumbnailImageUrl() {

    $repr = $this->getThumbnailRepresentation();

    if($repr){

      return $this->urlOf($repr);

    } else if($this->hasWebAppItem()) {

      return $this->webAppIcon($this->getWebAppUrl());

    }

    return null;

  }

  public function getWebAppUrl() {

    return $this->urlOf($this->web_app_representation);

  }

  public function hasWebAppItem(){

    return is_string($this->getWebAppUrl());

  }

  public function getFullSizeImageUrl() {

    if($this->full_size_representation){

      return $this->urlOf($this->full_size_representation);

    }

  }

  public function hasFullSizeImage(){

    return is_string($this->getFullSizeImageUrl());

  }

  /**

   * Provides the ULR to be used for showing the the item

   * in an overlay widget.

   *

   * This is currently limited to images only.

   *

   * @return string

   */

  public function getOverlayImageUrl(){

    if($this->hasFullSizeImage()){

      return $this->getFullSizeImageUrl();

    }

    return null;

  }

  /**

   * Provides the URL to be used for linking directly to the item.

   * Such links will open the item directly in the browser window.

   * WebApp urls are preferred over full-size-image urls.

   * The media item can be a web-app or image.

   *

   * @return mixed

   */

  public function getOpenItemUrl(){

    if($this->hasWebAppItem()){

      $url = $this->getWebAppUrl();

      if($this->urlIsAccessible($url) == self::$OK){

        return $url;

      }

    }

    if($this->hasFullSizeImage()){

      $url = $this->getFullSizeImageUrl();

      if($this->urlIsAccessible($url) == self::$OK){

        return $url;

      }

    }

    return null;

  }

  private function getThumbnailRepresentation() {

    if ($this->thumbnail_representation) {

      return $this->thumbnail_representation;

    } else {

      // use fallback strategy

      if ($this->full_size_representation) {

        return $this->full_size_representation;

      } else if(!$this->web_app_representation->uuid || $this->first_representation->uuid !== $this->web_app_representation->uuid){

        return $this->first_representation;

      }

    }

  }

  /**

   * @param $media_representation

   *  The CDM MediaRepresentation

   *

   * @return string|null

   *  The fist URL found in the representation parts or null

   */

  private function urlOf($media_representation){

    if(isset($media_representation->parts)){

      foreach($media_representation->parts as $part){

        if(isset($part->uri)){

          return $part->uri;

        }

      }

    }

    return null;

  }

  /**

   * @param $url

   *

   * @return false|mixed|string

   *

   * function found at https://stackoverflow.com/questions/5701593/how-to-get-a-websites-favicon-with-php#5702084

   */

  private function webAppIcon($url){

    $favicon = new FaviconDownloader($url);

    if ($favicon->icoExists) {

      return $favicon->icoUrl;

    }

    return null;

  }

  /**

   * @param $url

   *

   * @return int

   *  One of the GalleryItemMedia constants: $OK, $ACCESS_DENIED, $ERROR

   */

  private function urlIsAccessible($url){

    $result = drupal_http_request($url, ['method' => 'HEAD']);

    if($result->code == 200){

      return GalleryItemMedia::$OK;

    } else if($result->code >= 401 && $result->code <= 403) {

      return GalleryItemMedia::$ACCESS_DENIED;

    } else {

      return GalleryItemMedia::$ERROR;

    }

  }

  /**

   * @param $media_representation

   *

   * @return MediaSize object

   *    A MediaSize object with the fields

   *    - width: may be unset

   *    - height: may be unset

   *   or null if no size information is available

   */

  private function sizeOf($media_representation){

    if(isset($media_representation->parts[0])){

      $part = $media_representation->parts[0];

      if (isset_not_empty($part->width) || isset_not_empty($part->height)) {

        return new MediaSize(

          @$media_representation->parts[0]->width,

          @$media_representation->parts[0]->height

        );

      }

    }

    return null;

  }

}