How to ignore validation of some fields with condition in laravel

I have an OrderRequest with these rules:

public function rules()
    {
        return [
            //
            'surname' => 'required|max:255',
            'name' => 'required|max:255',
            'city' => 'required|max:255',
            'post_office'=>'required|max:255',
        ];
    }

I need to ignore surname and name validation if request has user_id

Currently doing it this way:

if ((Input::has('user_id'))) {
            return [
                //
                'city' => 'required|max:255',
                'post_office'=>'required|max:255',
            ];
        }
        return [
            //
            'surname' => 'required|max:255',
            'name' => 'required|max:255',
            'city' => 'required|max:255',
            'post_office'=>'required|max:255',
        ];
    }

But can I do it better way and how?

via Batmannn