code: private function readStreams(\Closure $callback = null): void
      {
          $stdOut = $this->readOutput(self::STDOUT);
          $stdErr = $this->readOutput(self::STDERR);
  
          $this->fullStdOut .= $stdOut;
          $this->fullStdErr .= $stdErr;
  
          if (!is_null($callback)) {
              $callback($stdOut, $stdErr);
          }
  
          $this->observer->stdOutRead($this->pid, $stdOut);
          $this->observer->stdErrRead($this->pid, $stdErr);
      }
docs: Read from the stdout & stderr streams, passing data to callback if provided.