customization - display featured image in RSS feed in WordPress

function add_featured_image_url($output) { global $post; if ( has_post_thumbnail( $post->
ID ) ){ $output = wp_get_attachment_url( get_post_thumbnail_id( $post->
ID ) ) . ' ' . $output; } return $output;
}
add_filter(he_excerpt_rss', 'add_featured_image_url');
add_filter(he_content_feed', 'add_featured_image_url');