Problem with Modelless Form inheritance in Cakephp4

I'm building a CommerceManager plugin to manage purchases for online shops.

CommerceManager plugin side

To deal with the billing/delivery step I have created a Modelless Form DeliveryForm which is useful for :

• Setting a list of countries available for delivery
• Validating the data of the billing/delivery form
• Writing the data in session (once it has been validated)
• Reading the billing/delivery data in session

// in plugin/CommerceManager/src/Form/DeliveryForm.php
class DeliveryForm extends Form
{
    /**
     * List of countries available used for <select> in delivery form
     * 
     * @var array
     */
    public $countries = [
    	'AL' => ['name' => 'ALBANIA', 'code' => '008'],
    	// [...]
    	'FR' => ['name' => 'FRANCE', 'code' => '250'],
    	// [...]
    ];

    public function validationDefault(Validator $validator): Validator
    {
        // validates form data
    }

    public function read()
    {
        $session = Session::create();
        return $session->read('Buyer') ?? [];
    }

	protected function _execute(array $data): bool
	{
	    $session = Session::create();
	    $session->write('Buyer', $data);

	    return true;
	}
}

My CommerceManager plugin has a CheckoutController which uses the plugin's DeliveryForm :

// in plugin/CommerceManager/src/Controller/CheckoutController.php
use CommerceManager\Form\DeliveryForm; // use plugin's DeliveryForm

class CheckoutController extends AppController
{
	public function delivery()
	{
		$deliveryForm = new DeliveryForm();
		if ($this->request->is('post')) {
			if ($deliveryForm->execute($this->request->getData())) {
				return $this->redirect(['action' => 'confirm']);
			}
		}
		else {
			$deliveryForm->setData($deliveryForm->read() ?? []);
		}

		$this->set(compact('deliveryForm'));
	}

	// [...]
}

App side

I would like to make a subclass of DeliveryForm for overriding the list of countries available for delivery.

To do that I've created a subclass of CheckoutController which only inherits the actions of its parent and uses App's DeliveryForm.

// in src/Controller/CheckoutController.php
use CommerceManager\Controller\CheckoutController as BaseCheckoutController;
use App\Form\DeliveryForm; // use App's DeliveryForm 

class CheckoutController extends BaseCheckoutController
{
	// inherits actions of CommerceManager.CheckoutController
}

// in src/Form/DeliveryForm.php
use CommerceManager\Form\DeliveryForm as BaseDeliveryForm;

class DeliveryForm extends BaseDeliveryForm
{
    public $countries = [
    	'FR' => ['name' => 'FRANCE', 'code' => '250'] // Only "France" is available
    ];
}

The problem comes from the fact that App still using CommerceManager's DeliveryForm.

I'm totally confused about how to deal with DeliveryForm inheritance...

How can I do for my App's CheckoutController could understand to use App's DeliveryForm subclass and not the plugin's one ?

You are misunderstanding how import resolution works, importing names does not "extend", it is a compilation time feature that only applies in the current file, see https://www.php.net/manual/en/language.namespaces.importing.php.

Basically at compilation time (before code actually runs) PHP will replace the references according to the imports in the file, and your app level class will then extend a plugin level class which has all its references already resolved, ie it will be like you're extending a class that has used fully qualified names everywhere, like:

$deliveryForm = new \CommerceManager\Form\DeliveryForm();

There's lots of different ways to solve your problem. It's hard for me to say what would be the best way, as it really depends on your application's specific requirements and architecture. You could for example set the config on the controller, and pass it to the form in the delivery() action accordingly. Or you could put the whole logic in a component, which you could configure in the extended controller accordingly. Or use an overridable form factory method on the controller to create the form instance. Or or or...

ps, avoid creating session instances manually, unless you're defining the session for the global request object, there should be no need to do that, ie do not use Session::create() in your form! Instead for example inject it as a dependency from the outside, like:

$deliveryForm = new DeliveryForm($this->request->getSession());

// in plugin/CommerceManager/src/Controller/CheckoutController.php
use CommerceManager\Form\DeliveryForm; // use plugin's DeliveryForm

class CheckoutController extends AppController
{
	/**
	 * The class to instanciate
	 */
	protected $_classnameDeliveryForm = \CommerceManager\Form\DeliveryForm::class;

	/**
	 * The instance
	 */
	protected $DeliveryForm;

	public function initialize(): void
    {
    	parent::initialize();
    	$this->DeliveryForm = new $this->_classnameDeliveryForm();
    }

    public function delivery()
    {
        if ($this->request->is('post')) {
            if ($this->DeliveryForm->execute($this->request->getData())) {
                return $this->redirect(['action' => 'confirm']);
            }
        }
        else {
            $this->DeliveryForm->setData($this->DeliveryForm->read() ?? []);
        }

        $this->set('deliveryForm', $this->DeliveryForm);
    }

    // [...]
}