think\Process::buildCallback PHP Method

    protected function buildCallback($callback)
    {
        $out = self::OUT;
        $callback = function ($type, $data) use($callback, $out) {
            if ($out == $type) {
                $this->addOutput($data);
            } else {
                $this->addErrorOutput($data);
            }
            if (null !== $callback) {
                call_user_func($callback, $type, $data);
            }
        };
        return $callback;
    }