bp_notifications_get_unread_notification_count( int $user_id )

function bp_notifications_get_unread_notification_count( $user_id = 0 ) {
	if ( empty( $user_id ) ) {
		$user_id = ( bp_displayed_user_id() ) ? bp_displayed_user_id() : bp_loggedin_user_id();
	}

	$count = wp_cache_get( $user_id, 'bp_notifications_unread_count' );
	if ( false === $count ) {
		$count = BP_Notifications_Notification::get_total_count( array(
			'user_id' => $user_id,
			'is_new'  => true,
		) );
		wp_cache_set( $user_id, $count, 'bp_notifications_unread_count' );
	}

	/**
	 * Filters the count of unread notification items for a user.
	 *
	 * @since BuddyPress 1.9.0
	 * @since BuddyPress 2.7.0 Added user ID parameter.
	 *
	 * @param int $count   Count of unread notification items for a user.
	 * @param int $user_id User ID for notifications count.
	 */
	return apply_filters( 'bp_notifications_get_total_notification_count', (int) $count, $user_id );
}