Application Architecture

Every CodeIgniter request enters through a single front controller, is matched to a route, passes through filters, and ends in a controller that builds a response. Understanding that path makes the folder layout much easier to remember.

The request lifecycle

1. public/index.php loads the framework and starts the request.
2. Configuration and services are initialised for the current environment.
3. The router matches the URL to a controller method.
4. Global and route filters run, for example authentication or CSRF checks.
5. The controller coordinates models, services, and validation.
6. A view or JSON response is returned to the browser.

Top-level folders

app/        Controllers, models, views, config, filters, helpers
public/     The web root: index.php and public assets
writable/   Logs, cache, sessions, uploads
vendor/     Composer dependencies
tests/      Automated tests
.env        Environment-specific settings

Only public should be exposed by the web server. Everything else stays outside the document root.

Inside the app folder

• Controllers handle requests and responses.
• Models represent tables and data rules.
• Views contain templates and partials.
• Config holds routes, filters, database settings, and application options.
• Filters run logic before or after controllers.
• Helpers contain small reusable functions.
• Libraries contain reusable classes that are not models.
• Database stores migrations and seeds.

Namespaces

Application classes use the App namespace. A file at app/Controllers/Admin/Reports.php declares namespace App\Controllers\Admin;. The folder path and namespace must match, or the autoloader will not find the class.

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