WordPress: Rekursiv nach Artikelbildern suchen

function get_featured_recursive($post) {
  if (has_post_thumbnail( $post->ID ) ) {
    return $post->ID;
  } else if ($post->post_parent != 0) {
    return get_featured_recursive(get_post($post->post_parent));
  } else {
    return null;
  }
}

if (wp_get_attachment_url( get_post_thumbnail_id(get_featured_recursive($post)) )!='') {
  $img = '<img src="' . wp_get_attachment_url( get_post_thumbnail_id(get_featured_recursive($post)) ) . '" alt="bild" />';
}