protected function isValidLuhn(string $number = null): bool
	{
		settype($number, 'string');

		$sumTable = [
			[
				0,
				1,
				2,
				3,
				4,
				5,
				6,
				7,
				8,
				9,
			],
			[
				0,
				2,
				4,
				6,
				8,
				1,
				3,
				5,
				7,
				9,
			],
		];

		$sum  = 0;
		$flip = 0;

		for ($i = strlen($number) - 1; $i >= 0; $i --)
		{
			$sum += $sumTable[$flip ++ & 0x1][$number[$i]];
		}

		return $sum % 10 === 0;
	}