Routing in CodeIgniter determines how URL requests are handled and which controller/method should process them. Instead of mapping URLs directly to file paths, CodeIgniter uses routes to decide which code should run. 

1. What is Routing in CodeIgniter?

When a user visits a URL, CodeIgniter’s routing system checks its app/Config/Routes.php file to determine which controller and method should handle the request.

https://example.com/blog/view/1

Here:

• blog → Controller name (Blog.php)
• view → Method name (view())
• 1 → Parameter passed to the method

2. Default Routing

By default, CodeIgniter uses this route:

$routes->get('/', 'Home::index');

• / means the home page.
• Home is the controller.
• index is the method.

3. Setting a Default Controller

If you want a different controller for the homepage:

$routes->get('/', 'Welcome::index');

Now visiting / will run Welcome controller’s index() method.

4. Passing Parameters in Routes

You can pass dynamic values to methods using placeholders.

$routes->get('product/(:num)', 'Product::view/$1');

• (:num) → matches only numbers.
• $1 → passes the matched value to the method.

5. Route Placeholders

CodeIgniter has several placeholders:

• (:num) → Numbers only
• (:alpha) → Alphabets only
• (:alphanum) → Alphabets + Numbers
• (:any) → Anything except a forward slash
• (:segment) → Any valid URI segment

6. HTTP Method-Specific Routes

You can restrict routes to specific HTTP methods:

$routes->post('submit-form', 'FormController::submit');
$routes->put('update/(:num)', 'Product::update/$1');
$routes->delete('delete/(:num)', 'Product::delete/$1');

7. Route Groups

Groups help organize related routes with a common prefix.

$routes->group('admin', function($routes) {
  $routes->get('dashboard', 'Admin\Dashboard::index');
  $routes->get('users', 'Admin\Users::index');
});

Now:

• /admin/dashboard
• /admin/users

Named Routes

You can name a route for easier URL generation:

$routes->get('login', 'Auth::login', ['as' => 'login']);

Then use:

url_to('login');