The CodeIgniter folder structure is given below:

• application: This directory will have your application logic. All of your application CODES will be held in this directory. Internal subdirectories in the CodeIgniter directory structure are given below:
  • cache – It stores cached files.
  • config – It keeps configuration files.
  • CONTROLLER – All application controllers are defined under this controller.
  • core – It consists of custom core classes that extend system files. For example, if you create a base controller that other controllers should extend, then you should place it under this directory.
  • helpers – This directory will be used for user-defined helper functions.
  • hooks – It is used for custom hooks in the CodeIgniter folder structure.
  • language – It is used to store language files for applications that use multiple languages.
  • libraries – It is used to store custom-created libraries.
  • logs – Application log files are placed in this directory.
  • models - All application models must be defined under this directory.
  • third_party – This is used for custom many packages that are created by you or other developers.
  • views – application views will be stored in this directory.
• system: It consists of the framework core files. It is not advised to MAKE any modifications in this directory or put your own application code into this directory. System subdirectories in CodeIgniter are given below:
  • core – This is considered to be the heart of the CodeIgniter Framework. All of the core files that construct the framework are located here. If you would like to extend the core file functionality, then you must
  • create a custom core file in the application directory. After this, you are allowed to override or add new behavior that you wish. You should never make any changes directly in this directory.
  • database – It stores the files such as database drivers, cache, and other files that are NEEDED for database operations.
  • fonts – This directory contains fonts and font-related information.
  • helpers – This directory consists of helper functions that come out of the box.
  • language – It contains language files that are used by the framework
  • libraries – It contains the source files for the different libraries that come ALONG with CodeIgniter out of the box.
• user_guide: This directory consists of a user manual for CodeIgniter. You should not upload this directory during application deployment.
• vendor: This directory consists of composer packages source code. The composer.json and composer.lock are the other two files related to this directory.
• index.php: This is considered as the entry point into the application. It is placed inside the root directory.