code: private function signatureHeader()
      {
          if ($signature = $this->fetchHeader('Signature')) {
              return $signature;
          } elseif ($authorization = $this->fetchHeader('Authorization')) {
              return substr($authorization, strlen('Signature '));
          } else {
              throw new Exception('HTTP message has no Signature or Authorization header');
          }
      }
docs: @return string
  
  @throws Exception