add-image-to-rss.php

<?php
/**
 * Add Image to RSS
 *
 * Source: https://wordpress.stackexchange.com/a/292763
 * Source: https://gist.github.com/DaveyJake/aa59667cd3ced5809b4d
 */
function remote_file_size($url)
{
	$data = get_headers($url, true);
	if (isset($data['Content-Length']))
		return (int) $data['Content-Length'];
}
add_action('rss2_item', function () {
	global $post;
	if (has_post_thumbnail($post->ID)) {
		$thumbnail_ID = get_post_thumbnail_id($post->ID);
		$thumbnail = wp_get_attachment_image_src($thumbnail_ID, 'article_thumb');
		$mime = get_post_mime_type($thumbnail_ID);
		if (is_array($thumbnail)) {
			echo '<enclosure url="' . $thumbnail[0] . '" length="' . remote_file_size($thumbnail[0]) . '" type="' . $mime . '" data-has-syndication-rights="1" data-portal-copyright="' . get_post_meta($thumbnail_ID, 'hlidacipes_licence_author', true) . '" />';
		}
	}
});