Yii - Overview

The Yii[ji:] framework is an open-source PHP framework for rapidly-developing, modern Web applications. It is built around the Model-View-Controller composite pattern.
Yii provides secure and professional features to create robust projects rapidly. The Yii framework has a component-based architecture and a full solid caching support.

Yii - Installation

The most straightforward way to get started with Yii2 is to use the basic application template provided by the Yii2 team. This template is also available through the Composer tool.
Step 1 − Find a suitable directory in your hard drive and download the Composer PHAR (PHP archive) via the following command.

Yii - Create Page

Now we are going to create a “Hello world” page in your application. To create a page, we must create an action and a view.
Actions are declared in controllers. The end user will receive the execution result of an action.
Step 1 − Declare the speak action in the existing SiteController, which is defined in the class file controllers/SiteController.php.

Yii - Application Structure

There is only one folder in the overall code base that is publicly available for the web server. It is the web directory. Other folders outside the web root directory are out of reach for the web server.
Note − All project dependencies are located in the composer.json file. Yii2 has a few important packages that are already included in your project by Composer. These packages are the following −

Yii - Entry Scripts

Entry scripts are responsible for starting a request handling cycle. They are just PHP scripts accessible by users.
The following illustration shows the structure of an application −

Yii - Controllers

Controllers are responsible for processing requests and generating responses. After user's request, the controller will analyze request data, pass them to model, then insert the model result into a view, and generate a response.

Yii - Using Controllers

Controllers in web applications should extend from yii\web\Controller or its child classes. In console applications, they should extend from yii\console\Controller or its child classes.
Let us create an example controller in the controllers folder.

Yii - Using Actions

To create an action in a controller class, you should define a public method whose name starts with the word action. The return data of an action represents the response to be sent to the end user.
Step 1 − Let us define the hello-world action in our ExampleController.

<?php 

Yii - Models

Models are objects representing business logic and rules. To create a model, you should extend the yii\base\Model class or its subclasses.

Attributes

Attributes represent the business data. They can be accessed like array elements or object properties. Each attribute is a publicly accessible property of a model. To specify what attributes a model possesses, you should override the yii\base\Model::attributes() method.
Let us have a look at the ContactForm model of the basic application template.

Yii - Widgets

A widget is a reusable client-side code, which contains HTML, CSS, and JS. This code includes minimal logic and is wrapped in a yii\base\Widget object. We can easily insert and apply this object in any view.
Step 1 − To see widgets in action, create an actionTestWidget function in the SiteController with the following code.

Yii - Modules

A module is an entity that has its own models, views, controllers, and possibly other modules. It is practically an application inside the application.
Step 1 − Create a folder called modules inside your project root. Inside the modules folder, create a folder named hello. This will be the basic folder for our Hello module.
Step 2 − Inside the hello folder, create a file Hello.php with the following code.

Yii - Views

Views are responsible for presenting the data to end users. In web applications, Views are just PHP script files containing HTML and PHP code.

Creating Views

Step 1 − Let us have a look at the ‘About’ view of the basic application template.

<?php
   /* @var $this yii\web\View */
   use yii\helpers\Html;
   $this->title = 'About';

Yii - Layouts

Layouts represent the common parts of multiple views i.e. for example, page header and footer. By default, layouts should be stored in the views/layouts folder.
Let us have a look at the main layout of the basic application template −

Yii - Assets

An asset is a file (css, js, video, audio or image, etc.) that may be referenced in a web page. Yii manages assets in asset bundles. The purpose of an asset bundle is to have a group of related JS or CSS files in the code base and to be able to register them within a single PHP call. Asset bundles can also depend on other asset bundles.

Yii - Asset Conversion

Instead of writing CSS or JS code, developers often use extended syntax, like LESS, SCSS, Stylus for CSS and TypeScript, CoffeeScript for JS. Then they use special tools to convert these files into real CSS and JS.

Yii - Extensions

Extensions are packages specifically designed to be used in Yii applications. You can share your own code as an extension or use third-party extensions to add features to your application.

Using Extensions

Most extensions are distributed as Composer packages. Composer installs packages from Packagist – the repository for Composer packages.
To install a third-party extension, you should −

Yii - Creating Extensions

Let us create a simple extension displaying a standard “Hello world” message. This extension will be distributed via the Packagist repository.
Step 1 − Create a folder called hello-world in your hard drive but not inside the Yii basic application template). Inside the hello-world directory, create a file named composer.json with the following code.

Yii - HTTP Requests

Requests are represented by the yii\web\Request object, which provides information about HTTP headers, request parameters, cookies, and so forth.
The methods get() and post() return request parameters of the request component.
Example −

Yii - Responses

When a web application handles a request, it generates a response object, which contains HTTP headers, body, and HTTP status code. In most cases, you will use the response application component. By default, it is an instance of yii\web\Response.

Yii - URL Formats

When a Yii application processes a requested URL, first, it parses the URL into a route. Then, to handle the request, this route is used to instantiate the corresponding controller action. This process is called routing. The reverse process is called URL creation. The urlManager application component is responsible for routing and URL creation. It provides two methods −