code: private function getIdPath(string $id):string
      {
          if (!preg_match("/^[a-z0-9]$/ui", $id)) {
              throw new HandlerException(
                  sprintf("The session id %s is invalid", $id),
                  $this
              );
          }
          return $this->savePath."/$id.session";
      }
docs: Returns the local file path for a given id.
  
  @param string $id
  @return string
  @throws HandlerException