WP_REST_Posts_Controller::handle_featured_media PHP Method

    protected function handle_featured_media($featured_media, $post_id)
    {
        $featured_media = (int) $featured_media;
        if ($featured_media) {
            $result = set_post_thumbnail($post_id, $featured_media);
            if ($result) {
                return true;
            } else {
                return new WP_Error('rest_invalid_featured_media', __('Invalid featured media ID.'), array('status' => 400));
            }
        } else {
            return delete_post_thumbnail($post_id);
        }
    }