<?php
declare(strict_types=1);

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

    public static function create(array $data): self;
}

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

/**
 * Class Payload
 */
class Payload implements IPayload {

    /**
     * @var string
     */
    private $varOne;

    /**
     * @param array $data
     * @return IPayload
     */
    public static function create(array $data): IPayload
    {
        $self = new self();
        $self->varOne = $data['varOne'] ?? null;
        return $self;
    }

    /**
     * @return array
     */
    public function toArray(): array {
        return ['varOne' => $this->varOne];
    }
}

/**
 * Class ChildPayload
 */
class ChildPayload extends Payload {

    /**
     * @var string
     */
    private $varTwo;

    /**
     * @param array $data
     * @return IPayload
     */
    public static function create(array $data): IPayload
    {
        $self = new self();
        $self->varTwo = $data['varTwo'] ?? null;
        return $self;
    }

    /**
     * @return array
     */
    public function toArray(): array {
        return ['varTwo' => $this->varTwo];
    }
}

/**
 * Class PayloadFactory
 */
class PayloadFactory implements IPayloadFactory {
    /**
     * @return IPayload
     */
    public function create(): IPayload  {
        return Payload::create(['varOne' => 'testOne']);
    }
}

/**
 * Class ChildPayloadFactory
 */
class ChildPayloadFactory implements IPayloadFactory {
    /**
     * @return IPayload
     */
    public function create(): IPayload  {
        return ChildPayload::create(['varTwo' => 'testTwo']);
    }
}

var_dump((new PayloadFactory())->create()->toArray());

var_dump((new ChildPayloadFactory())->create()->toArray());