CalculatablePrototype.php

<?php

abstract class Calculatable
{
    const SUBTRACT_ACTION = 'subtract';
    const SUBTRACT_SYMBOL = '-';
    const ADD_ACTION = 'add';
    const ADD_SYMBOL = '+';

    public function add(Calculatable $child): Calculatable
    {
        return $this->action($child, self::ADD_ACTION);
    }

    public function subtract(Calculatable $child): Calculatable
    {
        return $this->action($child, self::SUBTRACT_ACTION);
    }

    abstract protected function action(Calculatable $child, string $action): Calculatable;

    public function asFloat(): float
    {
        return $this->calculate();
    }

    public function describe(): string
    {
        return $this->buildFormula();
    }

    abstract protected function calculate(): float;

    abstract protected function buildFormula(): string;
}

class Calculation extends Calculatable
{

    private $firstElement;
    private $action;
    private $secondElement;

    public function __construct(Calculatable $firstElement, string $action, Calculatable $secondElement)
    {
        $this->firstElement = $firstElement;
        $this->action = $action;
        $this->secondElement = $secondElement;
    }

    protected function action(Calculatable $child, string $action): Calculatable
    {
        $calculation = new Calculation($this, $action, $child);

        return $calculation;
    }

    /**
     * @throws Exception
     */
    protected function getActionSymbol(): string
    {
        switch ($this->action) {
            case self::ADD_ACTION:
                return self::ADD_SYMBOL;
                break;
            case self::SUBTRACT_ACTION:
                return self::SUBTRACT_SYMBOL;
                break;
            default:
                throw new Exception("action {$this->action} was not implemented");
        }
    }

    /**
     * @throws Exception
     */
    protected function calculate(): float
    {
        $firstCalc = $this->firstElement->calculate();
        $secondCalc = $this->secondElement->calculate();
        if ($this->action == self::SUBTRACT_ACTION) {
            return $firstCalc - $secondCalc;
        } elseif ($this->action == self::ADD_ACTION) {
            return $firstCalc + $secondCalc;
        } else {
            throw new Exception("action {$this->action} was not implemented");
        }
    }

    /**
     * @throws Exception
     */
    protected function buildFormula(): string
    {
        $firstElement = $this->firstElement->buildFormula();
        $secondElement = $this->secondElement->buildFormula();
        $actionSymbol = $this->getActionSymbol();

        return "({$firstElement} {$actionSymbol} {$secondElement})";
    }
}

Class Money extends Calculatable
{
    private $value;

    public function __construct(float $value)
    {
        $this->value = $value;
    }

    public function getValue()
    {
        return $this->value;
    }

    protected function calculate(): float
    {
        return $this->getValue();
    }

    protected function action(Calculatable $child, string $action): Calculatable
    {
        return new Calculation($this, $action, $child);
    }

    protected function buildFormula(): string
    {
        return "{$this->getValue()}";
    }
}

function Money($value)
{
    return new Money($value);
}

$a = Money(10.0)->add(Money(20.0))->subtract(Money(5.0));
$b = Money(40.0)->subtract($a->subtract(Money(3.0)));
$c = Money(20)->add($b)->subtract($a);

echo $a->asFloat().PHP_EOL;  // 25.0
echo $a->describe().PHP_EOL; // ((10 + 20) – 5)
echo $b->asFloat().PHP_EOL;   // 18.0
echo $b->describe().PHP_EOL;  // (40.0 – (((10 + 20) - 5) – 3))
echo $c->asFloat().PHP_EOL; // 13
echo $c->describe().PHP_EOL; // ((20 + (40 - (((10 + 20) - 5) - 3))) - ((10 + 20) - 5))