Source Reference:

• Component: bbPress:: bbp-user

  File: /bbp-includes/bbp-user-functions.php :: Trac Source Line: 640

  		if ( !empty( $subscriptions ) ) {
  			$subscriptions = implode( ',', $subscriptions );
  			update_user_meta( $user_id, '_bbp_subscriptions', $subscriptions );
  		} else {
  			delete_user_meta( $user_id, '_bbp_subscriptions' );
  		}
  	}
  
  	do_action( 'bbp_remove_user_subscription', $user_id, $topic_id );
  
  	return true;
  }
  
  /**
   * Handles the front end subscribing and unsubscribing topics
   *
   * @uses bbp_is_subscriptions_active() To check if the subscriptions are active
   * @uses bbp_get_user_id() To get the user id
   * @uses current_user_can() To check if the current user can edit the user