code: protected function init($ftkey)
      {
          if (strlen($ftkey) != 32) {
              throw new Ts3Exception("invalid file transfer key format");
          }
  
          $this->getProfiler()->start();
          $this->getTransport()->send($ftkey);
  
          Signal::getInstance()->emit("filetransferHandshake", $this);
      }
docs: Sends a valid file transfer key to the server to initialize the file transfer.
  
  @param  string $ftkey
  @throws Ts3Exception
  @return void