code: public function getETags()
      {
          if ($this->headers->has('If-None-Match')) {
              return preg_split('/[\s,]+/', str_replace('-gzip', '', $this->headers->get('If-None-Match')), -1, PREG_SPLIT_NO_EMPTY);
          }
  
          return [];
      }
docs: Gets the Etags.
  
  @return array The entity tags