bbp_get_forum_ancestors( int $forum_id )

function bbp_get_forum_ancestors( $forum_id = 0 ) {
	$forum_id  = bbp_get_forum_id( $forum_id );
	$ancestors = array();
	$forum     = bbp_get_forum( $forum_id );

	if ( !empty( $forum ) ) {
		while ( 0 !== (int) $forum->post_parent ) {
			$ancestors[] = $forum->post_parent;
			$forum       = bbp_get_forum( $forum->post_parent );
		}
	}

	return apply_filters( 'bbp_get_forum_ancestors', $ancestors, $forum_id );
}