Yii is a very flexible and high-performance application development framework written in PHP. It helps building web applications, from small to large-scale enterprise applications. The framework name stands for Yes It Is (Yii).          Page: 5

Features fo Yii are : 
i. Model-View-Controller (MVC) design pattern
ii. Database Access Objects (DAO), Query Builder, Active Record, DB Migration
iii. Form input and validation
iv. AJAX-enabled widgets
v. Unit and functionality testing
vi. Friendly with third-party code

: Elaboration of MVC  is Model View Controller. It is a pattern for building application. The Model represents the application core (for instance a list of database records). The View displays the data (the database records) and The Controller is the business logic.

The first function that gets loaded from a controller is index(). 
example:
Public  function actionIndex()
{
 /* write code here*/
}

To connect a database, you have  to edit the database .php file which is at yii\protected\config\database.php:

<?php
	'connectionString' => 'mysql:host=localhost;dbname=yii116',
	'emulatePrepare' => true,
	'username' => 'root',
	'password' => '',
	'charset' => 'utf8',	
);