code: protected function internalFork(Process $p)
      {
          $p->setReady(false);
  
          $pid = pcntl_fork();
          if (-1 === $pid) {
              throw new \RuntimeException('Failure on pcntl_fork');
          }
  
          if ($pid) {
              $this->children[$pid] = $p;
  
              return $p->setPid($pid)
                  ->waitReady();
          }
  
          $p->setPid(getmypid())
              ->run();
  
          exit(0);
      }
docs: @param Process $p
  
  @return Process