bbp_get_user_subscribed_forum_ids( int $user_id )

function bbp_get_user_subscribed_forum_ids( $user_id = 0 ) {
	$user_id = bbp_get_user_id( $user_id );
	if ( empty( $user_id ) )
		return false;

	$subscriptions = get_user_option( '_bbp_forum_subscriptions', $user_id );
	$subscriptions = array_filter( wp_parse_id_list( $subscriptions ) );

	return (array) apply_filters( 'bbp_get_user_subscribed_forum_ids', $subscriptions, $user_id );
}