code: protected function validateCache($lastModified, $etag)
      {
          if (isset($_SERVER['HTTP_IF_NONE_MATCH'])) {
              // HTTP_IF_NONE_MATCH takes precedence over HTTP_IF_MODIFIED_SINCE
              // http://tools.ietf.org/html/rfc7232#section-3.3
              return $etag !== null && in_array($etag, Yii::$app->request->getEtags(), true);
          } elseif (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
              return $lastModified !== null && @strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $lastModified;
          } else {
              return $etag === null && $lastModified === null;
          }
      }
docs: Validates if the HTTP cache contains valid content.
  @param integer $lastModified the calculated Last-Modified value in terms of a UNIX timestamp.
  If null, the Last-Modified header will not be validated.
  @param string $etag the calculated ETag value. If null, the ETag header will not be validated.
  @return boolean whether the HTTP cache is still valid.