Tax information removed in taxed money calculations

[rvdsteege]

When performing calculations in a TaxedMoney object, the tax details are removed.

$taxed = new TaxedMoney( 10, 'EUR', 2.10 );

var_dump( $taxed->get_tax_value() ); // string '2.1'

var_dump( $taxed->multiply( 5 )->get_tax_value() ); // null

This is caused by the fact that we're instantiating a new static (either Money or TaxedMoney) object from e.g. the multiply() method.

wp-money/src/Money.php

Lines 328 to 344 in 6b144b3

	/**
	* Returns a new Money object that represents
	* the multiplied value of this Money object.
	*
	* @link https://github.com/moneyphp/money/blob/v3.2.1/src/Money.php#L299-L316
	*
	* @param mixed $multiplier Multiplier.
	*
	* @return Money
	*/
	public function multiply( $multiplier ) {
	$multiplier = Number::from_mixed( $multiplier );
	$result = $this->amount->multiply( $multiplier );
	return new static( $result, $this->currency );
	}

Also, PHPStan currently warns that the method signature of TaxedMoney::__construct() is not enforced to be equal to the Money constructor signature:

Unsafe usage of new static().
💡 See: https://phpstan.org/blog/solving-phpstan-error-unsafe-usage-of-new-static

Both issues can be resolved by introducing a MoneyInterface class, which needs to be implemented by both the Money and TaxedMoney classes. The interface should contain abstract methods for the constructor and calculation methods.

The multiply() method is currently in use in Gravity Forms subscription period alignment, where losing tax details is fine as we're not specifying taxes for the payment lines in this integration.