bbp_add_user_forum_subscription( int $user_id, int $forum_id )

function bbp_add_user_forum_subscription( $user_id = 0, $forum_id = 0 ) {
	if ( empty( $user_id ) || empty( $forum_id ) ) {
		return false;
	}

	$forum = bbp_get_forum( $forum_id );
	if ( empty( $forum ) ) {
		return false;
	}

	$subscriptions = (array) bbp_get_user_subscribed_forum_ids( $user_id );
	if ( !in_array( $forum_id, $subscriptions ) ) {
		$subscriptions[] = $forum_id;
		$subscriptions   = implode( ',', wp_parse_id_list( array_filter( $subscriptions ) ) );
		update_user_option( $user_id, '_bbp_forum_subscriptions', $subscriptions );

		wp_cache_delete( 'bbp_get_forum_subscribers_' . $forum_id, 'bbpress_users' );
	}

	do_action( 'bbp_add_user_forum_subscription', $user_id, $forum_id );

	return true;
}