code: protected function isWhitespace(&$length)
      {
          static $whitespaces = [
              "\f" => true,
              "\n" => true,
              "\r" => true,
              "\t" => true,
              ' ' => true,
          ];
  
          $length = 1;
          return isset($whitespaces[$this->substring($length)]);
      }
docs: {@inheritdoc}