CodeIgniter Internals Overview

This guide should help contributors understand how the core of the framework works, and what needs to be done when creating new functionality. Specifically, it details the information needed to create new packages for the core.


All packages should be designed to be completely isolated from the rest of the packages. This will allow them to be used in projects outside of CodeIgniter. Basically, this means that all dependencies should be kept to a minimum. Any dependencies must be able to be passed into the constructor. If you do need to use one of the other core packages, you can create that in the constructor using the Services class, as long as you provide a way for dependencies to override that:

public function __construct(Foo $foo=null)
        $this->foo = $foo instanceOf Foo
                ? $foo
                : \Config\Services::foo();

Type hinting

PHP7 provides the ability to type hint method parameters and return types. Use it where possible. Return type hinting is not always practical, but do try to make it work.

At this time, we are not using strict type hinting.


The amount of abstraction required to implement a solution should be the minimal amount required. Every layer of abstraction brings additional levels of technical debt and unnecessary complexity. That said, don't be afraid to use it when it's needed and can help things.

  • Don't create a new container class when an array will do just fine.
  • Start simple, refactor as necessary to achieve clean separation of code, but don't overdo it.


Any new packages submitted to the framework must be accompanied by unit tests. The target is 80%+ coverage of all classes within the package.

  • Test only public methods, not protected and private unless the method really needs it due to complexity.
  • Don't just test that the method works, but test for all fail states, thrown exceptions, and other pathways through your code.

Namespaces and Files

All new packages should live under the CodeIgniter namespace. The package itself will need its own sub-namespace that collects all related files into one grouping, like CodeIgniter\HTTP.

Files MUST be named the same as the class they hold, and they must match the Style Guide, meaning CamelCase class and file names. The should be in their own directory that matches the sub-namespace under the system directory.

The the Router as an example. The Router lives in the CodeIgniter\Router namespace. It has two classes, RouteCollection and Router, which are in the files, system/Router/RouteCollection.php and system/Router/Router.php respectively.


Most base classes should have an interface defined for them. At the very least this allows them to be easily mocked and passed in other classes as a dependency without breaking the type-hinting. The interface names should match the name of the class with "Interface" appended to it, like RouteCollectionInterface.

The Router package mentioned above includes the CodeIgniter\Router\RouterCollectionInterface and CodeIgniter\Router\RouterInterface interfaces to provide the abstractions for the two classes in the package.


When a package supports multiple "drivers", the convention is to place them in a Handlers directory, and name the child classes as Handlers. You will often find that creating a BaseHandler the child classes can extend to be beneficial in keeping the code DRY.

See the Log and Session packages for examples.


Should the package require user-configurable settings, you should create a new file just for that package under application/Config. The file name should generally match the package name.


All files within the package should be added to system/Config/AutoloadConfig.php, in the "classmap" property. This is only used for core framework files, and helps to minimize file system scans and keep performance high.

Command-Line Support

CodeIgniter has never been known for it's strong CLI support. However, if your package could benefit from it, create a new file under system/Commands. The class contained within is simply a controller that is intended for CLI usage only. The index() method should provide a list of available commands provided by that package.

Routes must be added to system/Config/Routes.php using the cli() method to ensure it is not accessible through the browser, but is restricted to the CLI only.

See the MigrationsCommand file for an example.


All packages must contain appropriate documentation that matches the tone and style of the rest of the user guide. In most cases, the top portion of the package's page should be treated in tutorial fashion, while the second half would be a class reference.