Yii and CodeIgniter both follow MVC or Model-View-Controller Structure, but Yii has a strict name convention for files. Yii supports CRUD Generation using Gii, but CodeIgniter does not support CRUD generation. CodeIgniter has a PHPUnit as a testing library, but Yii has a PHPUnit and Selenium as a testing library

CodeIgniter is less feature-rich and advanced than Yii, which can make it less suitable for complex and large-scale applications. CodeIgniter also has a less powerful and expressive ORM than Yii, which can make it more difficult to work with databases.