DesignPatternsPHP/More/Repository/Domain/PostId.php

<?php
declare(strict_types=1);

namespace DesignPatterns\More\Repository\Domain;

/**
 * This is a perfect example of a value object that is identifiable by it's value alone and
 * is guaranteed to be valid each time an instance is created. Another important property of value objects
 * is immutability.
 *
 * Notice also the use of a named constructor (fromInt) which adds a little context when creating an instance.
 */
class PostId
{
    /**
     * @var int
     */
    private $id;

    public static function fromInt(int $id)
    {
        self::ensureIsValid($id);

        return new self($id);
    }

    private function __construct(int $id)
    {
        $this->id = $id;
    }

    public function toInt(): int
    {
        return $this->id;
    }

    private static function ensureIsValid(int $id)
    {
        if ($id <= 0) {
            throw new \InvalidArgumentException('Invalid PostId given');
        }
    }
}