code: public function destroy()
      {
          if ($this->getIsActive()) {
              $sessionId = session_id();
              $this->close();
              $this->setId($sessionId);
              $this->open();
              session_unset();
              session_destroy();
              $this->setId($sessionId);
          }
      }
docs: Frees all session variables and destroys all data registered to a session.
  
  This method has no effect when session is not [[getIsActive()|active]].
  Make sure to call [[open()]] before calling it.
  @see open()
  @see isActive