正如我所看到的，如果我们想实例化一个模型（例如命名Post），我们只需要调用：
$post = new Post();
现在，我还想实例化一个Controller（例如 namedPost和这个控制器的 php 文件PostController.php）。所以我使用这段代码：
$postController = new PostController();
但是，运行此代码时出现错误。
我做了一些搜索，发现实例化应该如下所示：
$postController = Yii::app()->createController('post/index');
它运行正确。但我仍然想知道为什么第一种方法不起作用？
Answering your exact question "why the first approach doesn't work".
The folder /protected/controller is NOT in "include path" of the project.
Just add 'import'=>array('application.controllers.*') into your config file or use
include(Yii::app()->getBasePath().DIRECTORY_SEPARATOR.'controllers'.DIRECTORY_SEPARATOR.'PostController.php');
just before creating an object of PostController. Ah and creating new controller requires a name for this controller, so it should be something like
$controller = new PostController('post_controller');
I would like to point out that this type of controller creation is useless in Yii, as you are creating a controller completely separated from project, so it will be almost useless. As you noted, the correct way to create controller is through Yii::app()->createController()
只需使用：
$controller = Yii::app()->controller;
这将返回您当前的控制器以供请求。
另请参阅此处的文档：http ://www.yiiframework.com/doc/api/1.1/CApplication#controller-detail