code: public function signal(int $signo)
      {
          if (!$this->isRunning()) {
              throw new StatusError("Process is not running.");
          }
  
          $this->processRunner->signal($this->handle, $signo);
      }
docs: Send a signal signal to the process.
  
  @param int $signo Signal number to send to process.
  
  @throws StatusError If the process is not running.
  @throws ProcessException If sending the signal fails.