foo.php

<?php

interface Price
{
    public function money(): Money;
    public function currency(): Currency;
}


class Vat
{
  public function vatAmount(): VatAmount    
  public function vatCountry(): VatCountry
  public function vatRate(): VatRate
}

class VatAmount implements Price
{
    public function money(): Money;
    public function currency(): Currency;
}


// --- Considered bad practice?

class Vat implements Price
{
    public function money(): Money
    {
      return $this->vatAmount->money();
    }
  
    public function currency(): Currency
    {
      return $this->vatAmount->currency();
    }
}