Money.php

<?php

class Money
{
    private const DECIMAL_SCALE = 2;

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

    private function __construct(string $value)
    {
        $this->value = self::normalize($value);
    }

    public static function createZero(): self
    {
        return new self('0');
    }

    public static function createFromString(string $value): self
    {
        return new self($value);
    }

    public function toString(): string
    {
        return $this->value;
    }

    public function add(Money $amount): Money
    {
        return self::createFromString(bcadd($this->value, $amount->value, self::DECIMAL_SCALE));
    }

    public function substract(Money $amount): Money
    {
        return self::createFromString(bcsub($this->value, $amount->value, self::DECIMAL_SCALE));
    }

    public function isZero(): bool
    {
        return $this->equals(self::createZero());
    }

    public function invertSign(): Money
    {
        $zero = self::createZero();
        return $zero->substract($this);
    }

    private static function normalize(string $value): string
    {
        if (($value === '') || !preg_match('/^(-)?[0-9]+(\\.[0-9]+)?$/', $value)) {
            throw new \LogicException("Неподдерживаемый формат денег: {$value}");
        }

        return bcadd('0', $value, self::DECIMAL_SCALE);
    }

    private function equals(Money $money): bool
    {
        return bccomp($this->value, $money->value, self::DECIMAL_SCALE) === 0;
    }
}