OtpAlgorithm::generateOTP

public function generateOTP($counter)
{
	$hash = hash_hmac($this->getDigest(), static::toByte($counter), $this->getSecret());
	$hmac = [];
	foreach (str_split($hash, 2) as $hex)
	{
		$hmac[] = hexdec($hex);
	}

	$offset = $hmac[count($hmac) - 1] & 0xf;
	$code = ($hmac[$offset] & 0x7F) << 24;
	$code |= ($hmac[$offset + 1] & 0xFF) << 16;
	$code |= ($hmac[$offset + 2] & 0xFF) << 8;
	$code |= ($hmac[$offset + 3] & 0xFF);

	$otp = $code % pow(10, $this->getDigits());
	return str_pad($otp, $this->getDigits(), '0', STR_PAD_LEFT);
}