为什么使用路由参数的时候抛出 NotFoundHttpException 异常。

在没有使用路由参数的时候能正常显示：

Route::get('/',function (){ return view('welcome'); });

访问 http://localhost/laravel/public/ 的时候显示正常。
但是改为：

Route::get('user/{id?}',function ($id="1") { return view('welcome'); });

访问 http://localhost/laravel/public/user/ 的时候就跳转到 http://localhost/user 当然 Object not found!

并且访问 http://localhost/laravel/public/user/22 就抛出 NotFoundHttpException 异常：

Sorry, the page you are looking for could not be found.

1/1 NotFoundHttpException in compiled.php line 8157:

1. in compiled.php line 8157
2. at RouteCollection->match(object(Request)) in compiled.php line 7396
3. at Router->findRoute(object(Request)) in compiled.php line 7361
4. at Router->dispatchToRoute(object(Request)) in compiled.php line 7353
5. at Router->dispatch(object(Request)) in compiled.php line 2262
6. at Kernel->Illuminate\Foundation\Http{closure}(object(Request))
7. at call_user_func(object(Closure), object(Request)) in compiled.php line 9472
8. at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in compiled.php line 2886
9. at VerifyCsrfToken->handle(object(Request), object(Closure))
10. at call_user_func_array(array(object(VerifyCsrfToken), 'handle'), array(object(Request), object(Closure))) in compiled.php line 9464
11. at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in compiled.php line 12789
12. at ShareErrorsFromSession->handle(object(Request), object(Closure))
13. at call_user_func_array(array(object(ShareErrorsFromSession), 'handle'), array(object(Request), object(Closure))) in compiled.php line 9464
14. at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in compiled.php line 11412
15. at StartSession->handle(object(Request), object(Closure))
16. at call_user_func_array(array(object(StartSession), 'handle'), array(object(Request), object(Closure))) in compiled.php line 9464
17. at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in compiled.php line 12530
18. at AddQueuedCookiesToResponse->handle(object(Request), object(Closure))
19. at call_user_func_array(array(object(AddQueuedCookiesToResponse), 'handle'), array(object(Request), object(Closure))) in compiled.php line 9464
20. at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in compiled.php line 12469
21. at EncryptCookies->handle(object(Request), object(Closure))
22. at call_user_func_array(array(object(EncryptCookies), 'handle'), array(object(Request), object(Closure))) in compiled.php line 9464
23. at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in compiled.php line 2936
24. at CheckForMaintenanceMode->handle(object(Request), object(Closure))
25. at call_user_func_array(array(object(CheckForMaintenanceMode), 'handle'), array(object(Request), object(Closure))) in compiled.php line 9464
26. at Pipeline->Illuminate\Pipeline{closure}(object(Request))
27. at call_user_func(object(Closure), object(Request)) in compiled.php line 9454
28. at Pipeline->then(object(Closure)) in compiled.php line 2209
29. at Kernel->sendRequestThroughRouter(object(Request)) in compiled.php line 2192
30. at Kernel->handle(object(Request)) in index.php line 54

环境：Apache 5.6.12 Laravel 5