code: private function convertString($string)
      {
          // chop away beggining and ending quotes
          $string = substr($string, 1, -1);
          if (strlen($string) > 255) {
              throw new WriterException('String is too long');
          }
  
          return pack('C', $this->ptg['ptgStr']) . StringHelper::UTF8toBIFF8UnicodeShort($string);
      }
docs: Convert a string token to ptgStr.
  
  @param string $string a string for conversion to its ptg value
  
  @return mixed the converted token on success