src/filters/HttpCache.php
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', $this->cacheControlHeader);
        }
    }
protected function sendCacheControlHeader()
    {
        if ($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);
        }
    }
Sends the cache control header to the client
@see cacheControlHeader