Yii is a Model View Controller (MVC) Framework. It started in 2008 as an enhanced version of the PRADO PHP Framework. The main focus of Yii is that of speed, but security has been a growing focus as the Yii framework is continually developed.

Yii is popular with developers for several reasons:

1. A focus on security.

Yii comes with some basic security functions that include the ability to validate inputs and filter outputs, and prevention of cross-site scripting and SQL injection.

2. Ease of testing and compatibility.

Yii can be tested using either the PHPUnit or Selenium Unit testing frameworks. It is compatible with Zend Framework Code, as well as some other frameworks.

3. Open source availability.

Yii is open source, meaning that the framework is freely available for developers to use without incurring license fees or without having to purchase the Yii framework code itself.

4. Custom setup and applications.

When first setting up Yii, a developer sets up the Database and the CRUD (Create, Read, Update, Delete) first. The database and the CRUD may be set up without having to write any custom code. From there the developer would write code over top of the base database and CRUD to create specific custom applications of the base Yii Framework. All code generated by Yii components is in compliance with standard XHTML.