<?php
declare(strict_types=1);

/**
 * Interface IPayload
 */
interface IPayload {
    public function toArray(): array;
}

/**
 * Interface ChildPayloadInterface
 */
interface ChildPayloadInterface {
    public function setVar(string $var);
}

/**
 * Interface GenericPayload
 */
interface GenericPayload extends IPayload, ChildPayloadInterface {

}

/**
 * Interface IPayloadFactory
 */
interface IPayloadFactory {
    public function create(): IPayload;
}

/**
 * Interface IChildPayloadFactory
 */
interface IChildPayloadFactory {
    public function create(): GenericPayload;
}

/**
 * Class Payload
 */
class Payload implements IPayload {
    public function toArray(): array {
        return ['hello' => 'world'];
    }
}

/**
 * Class ChildPayload
 */
class ChildPayload extends Payload implements GenericPayload {
    public function setVar(string $var)  {
        $this->var = $var;
    }
}

/**
 * Class ChildPayloadFactory
 */
class ChildPayloadFactory implements IChildPayloadFactory {
    public function create(): GenericPayload  {
        return new ChildPayload();
    }
}


$factory = new ChildPayloadFactory();

var_dump($factory->create());