PhoneNumber.php

<?php 

namespace Acme\Project\Model\User\Value;

use Acme\Project\Model\Core\Validation\Assert;

final class PhoneNumber
{
    /**
     * @var string
     */
    private $value;

    /**
     * @param string $phoneNumber
     */
    public function __construct($phoneNumber)
    {
        Assert::string($phoneNumber, "Invalid phone number.");

        $this->value = $phoneNumber;
    }

    /**
     * @return string
     */
    public function __toString()
    {
        return $this->value ?: ''; // <-- important, Doctrine always creates the embeddables even if their value is null
    }
}