1-conclusion.php

public function addPhoneNumber(string $phone): void {}

1-doctrine-price.php

<?php declare(strict_types=1);

/** @Embeddable */
final class Price
{
    /** @Column(type="float") */
    private $amount;

    /** @Column(type="string") */
    private $currency;

    public function __construct(float $amount, string $currency = 'USD')
    {
        // ...

        $this->amount = $amount;
        $this->currency = $currency;
    }

    // ...
}

/** @Entity */
class Product
{
    /** @Embedded(class="Price") */
    private $price;

    public function __construct(Price $price)
    {
        $this->price = $price;
    }
}

1-immutability-price.php

<?php declare(strict_types=1);

final class Price
{
    // ...

    private function hasSameCurrency(Price $price): bool
    {
        return $this->currency === $price->currency;
    }

    public function sum(Price $price): self
    {
        if (!$this->hasSameCurrency($price)) {
            throw \InvalidArgumentException(
                "You can only sum values with the same currency: {$this->currency} !== {$price->currency}."
            );
        }

        return new self($this->amount + $price->amount, $this->currency);
    }
}

1-price.php

<?php declare(strict_types=1);

final class Price
{
  const USD = 'USD';
  const CAD = 'CAD';

  /** @var float */
  private $amount;

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

  public function __construct(float $amount, string $currency = 'USD')
  {
      if ($amount < 0) {
          throw new \InvalidArgumentException("Amount should be a positive value: {$amount}.");
      }

      if (!in_array($currency, $this->getAvailableCurrencies())) {
          throw new \InvalidArgumentException("Currency should be a valid one: {$currency}.");
      }

      $this->amount = $amount;
      $this->currency = $currency;
  }

  private function getAvailableCurrencies(): array
  {
      return [self::USD, self::CAD];
  }

  public function getAmount(): float
  {
      return $this->amount;
  }

  public function getCurrency(): string
  {
      return $this->currency;
  }
}

1-self-validation-price.php

<?php declare(strict_types=1);

final class Price
{
    // ...

    public function __construct(float $amount, string $currency = 'USD')
    {
        if ($amount < 0) {
            throw new \InvalidArgumentException("Amount should be a positive value: {$amount}.");
        }

        if (!in_array($currency, $this->getAvailableCurrencies())) {
            throw new \InvalidArgumentException("Currency should be a valid one: {$currency}.");
        }

        $this->amount = $amount;
        $this->currency = $currency;
    }
}

1-structural-equality-price.php

<?php declare(strict_types=1);

final class Price
{
    // ...

    public function isEqualsTo(Price $price): bool
    {
        return $this->amount === $price->amount && $this->currency === $price->currency;
    }
}

2-conclusion.php

public function addPhoneNumber(PhoneNumber $phone): void {}

2-structural-equality-price.php

<?php declare(strict_types=1);

final class Price
{
    // ...

    private function hash(): string
    {
        return md5("{$this->amount}{$this->currency}");
    }

    public function isEqualsTo(Price $price): bool
    {
        return $this->hash() === $price->hash();
    }
}