bbp_update_topic_forum_id( int $topic_id, int $forum_id )

function bbp_update_topic_forum_id( $topic_id = 0, $forum_id = 0 ) {

	// If it's a reply, then get the parent (topic id)
	if ( bbp_is_reply( $topic_id ) ) {
		$topic_id = bbp_get_reply_topic_id( $topic_id );
	} else {
		$topic_id = bbp_get_topic_id( $topic_id );
	}

	if ( empty( $forum_id ) ) {
		$forum_id = get_post_field( 'post_parent', $topic_id );
	}

	update_post_meta( $topic_id, '_bbp_forum_id', (int) $forum_id );

	return apply_filters( 'bbp_update_topic_forum_id', (int) $forum_id, $topic_id );
}