Finished the "Saying Hello" section [skip ci]

docs/guide/README.md

@@ -20,7 +20,7 @@ Getting Started
 
 * [Installing Yii](start-installation.md)
 * [Running Applications](start-workflow.md)
-* [Saying "Hello World"](start-hello-world.md)
+* [Saying Hello](start-hello.md)
 * [Working with Forms](start-forms.md)
 * [Working with Databases](start-databases.md)
 * [Generating Code with Gii](start-gii.md)

docs/guide/start-databases.md

+Working with Databases
+======================
+
+> Note: This section is under development.

docs/guide/start-forms.md

+Working with Forms
+==================
+
+> Note: This section is under development.

docs/guide/start-gii.md

+Generating Code with Gii
+========================
+
+> Note: This section is under development.

docs/guide/start-hello.md

+Saying Hello
+============
+
+In this section, we will describe how to add to your application a new page that displays "Hello".
+To achieve this goal, you will create an [action](structure-controllers.md) as well as
+a [view](structure-views.md). Yii dispatches the page request to the action which in turn
+renders the view with "Hello" to the user.
+
+Through this tutorial, you will learn
+
+* How to create an [action](structure-controllers.md) to respond to requests;
+* How to create a [view](structure-views.md) to compose response content;
+* How an application dispatches requests to [actions](structure-controllers.md).
+
+
+Creating an Action
+------------------
+
+[Actions](structure-controllers.md) are the only objects that end users can directly refer to and request
+for execution. Actions are grouped by [controllers](structure-controllers.md). The execution result of
+an action is the response that an end user will receive.
+
+For the "Hello" task, you will create a `say` action which reads a `message` parameter from
+a request and displays the message content back to the user. If the `message` parameter is not given,
+it will use the default value "Hello". For simplicity, you may put this action in an existing
+controller `SiteController` which is defined in the class file `controllers/SiteController.php`:
+
+```php
+class SiteController extends Controller
+{
+    // ...existing code...
+
+    public function actionSay($message = 'Hello')
+    {
+        return $this->render('say', ['message' => $message]);
+    }
+}
+```
+
+In the above code, the `say` action is defined as a method named `actionSay` in `SiteController`.
+Yii uses the prefix `action` to differentiate action methods from non-action methods in the class.
+The name after the `action` prefix is treated as the ID of the corresponding action.
+
+> Info: Actions defined by action methods are called *inline actions*. Yii will create an [[yii\base\InlineAction|InlineAction]]
+  object during runtime which will call the corresponding action method to handle a request.
+
+The action method takes a parameter `$message` which defaults to `"Hello"`. When the application
+receives a request and determines the `say` action is responsible to handle the request, it will
+populate this parameter with the same named parameter found in the request.
+
+Within the action method, the [[yii\web\Controller::render()|render()]] method is called which
+renders a [view](structure-views.md) named `say` and passes along the `message` parameter. The rendering
+result is returned by the action method, which will be taken by the application and displayed to the end user.
+
+
+Creating a View
+---------------
+
+[Views](structure-views.md) are scripts that you write to compose response content.
+For the "Hello" task, you will create a `say` view which echoes the `message` parameter
+passed from the `say` action when it calls `render()`:
+
+```php
+<?php
+use yii\helpers\Html;
+?>
+<?= Html::encode($message) ?>
+```
+
+Note that in the above code, the `message` parameter is [[yii\helpers\Html::encode()|HTML-encoded]]
+before being echoed. This is necessary because the parameter is coming from end users who may attempt
+[cross-site scripting (XSS) attacks](http://en.wikipedia.org/wiki/Cross-site_scripting) by embedding
+malicious JavaScript code in the parameter.
+
+The `say` view should be saved in the file `views/site/say.php`. When the method [[yii\web\Controller::render()|render()]]
+is called in an action, it will look for a PHP file named as `views/ControllerID/ActionID/ViewName.php`.
+
+> Tip: You may put more content in the `say` view. They can be HTML tags, plain text, or even PHP statements.
+  In fact, the `say` view is just a PHP script which is executed by the [[yii\web\Controller::render()|render()]] method.
+  The content echoed by the view script will be forwarded by the application as the response to the end user.
+
+
+How It Works
+------------
+
+After creating the action and the view, you may access the new page by the following URL:
+
+```
+http://hostname/index.php?r=site/say&message=Hello+World
+```
+
+This will show a page displaying "Hello World". The page shares the same header and footer as other pages of
+the application. If you omit the `message` parameter in the URL, you would see the page displays "Hello".
+This is because `message` is passed as a parameter to the `actionSay()` method, and when it is omitted,
+the default value of `"Hello"` will come into play.
+
+The `r` parameter requires more explanation. It stands for [route](runtime-routing.md) which is a globally unique ID
+referring to an action. Its format is `ControllerID/ActionID`. When the application receives
+a request, it will check this parameter and use the `ControllerID` part to determine which controller
+class should be instantiated to handle the request. Then, the controller will use the `ActionID` part
+to determine which action should be instantiated to do the real work. In our case, the route `site/say`
+will be resolved into the `SiteController` controller class and the `say` action. As a result,
+the `SiteController::actionSay()` method will be called to handle the request.
+
+
+Summary
+-------
+
+In this section, you have touched the controller part and the view part in the MVC design pattern.
+You created an action as part of a controller to handle requests. And you also created a view script
+to compose response content. There is no model involved in this task because the only data used is
+the simple `message` parameter.
+
+In the next section, you will get into touch with the model part through building HTML forms.

docs/guide/start-looking-head.md

+Looking Ahead
+=============
+
+> Note: This section is under development.