get_post_ancestors()

get_post_ancestors( int|WP_Post $post ): int[]

function get_post_ancestors( $post ) {
	$post = get_post( $post );

	if ( ! $post || empty( $post->post_parent ) || $post->post_parent === $post->ID ) {
		return array();
	}

	$ancestors = array();

	$id          = $post->post_parent;
	$ancestors[] = $id;

	while ( $ancestor = get_post( $id ) ) {
		// Loop detection: If the ancestor has been seen before, break.
		if ( empty( $ancestor->post_parent ) || $ancestor->post_parent === $post->ID
			|| in_array( $ancestor->post_parent, $ancestors, true )
		) {
			break;
		}

		$id          = $ancestor->post_parent;
		$ancestors[] = $id;
	}

	return $ancestors;
}