code: private function headerValue($name)
      {
          if ($this->message->hasHeader($name)) {
              $header = $this->message->getHeader($name);
  
              return end($header);
          } else {
              throw new SignedHeaderNotPresentException("Header '$name' not in message");
          }
      }
docs: @param string $name
  
  @return string
  
  @throws SignedHeaderNotPresentException