code: protected function sendCacheControlHeader()
      {
          if ($this->sessionCacheLimiter !== null) {
              if ($this->sessionCacheLimiter === '' && !headers_sent() && Yii::$app->getSession()->getIsActive()) {
                  header_remove('Expires');
                  header_remove('Cache-Control');
                  header_remove('Last-Modified');
                  header_remove('Pragma');
              }
              session_cache_limiter($this->sessionCacheLimiter);
          }
  
          $headers = Yii::$app->getResponse()->getHeaders();
          $headers->set('Pragma');
  
          if ($this->cacheControlHeader !== null) {
              $headers->set('Cache-Control', $this->cacheControlHeader);
          }
      }
docs: Sends the cache control header to the client
  @see cacheControlHeader