PHP不使用速算扣除数计算个人所得税

/**
     * PHP不使用速算扣除数计算个人所得税
     * @param $salary float 含税收入金额
     * @param int $deduction float $deduction 保险等应当扣除的金额 默认值为0
     * @param int $threshold float $threshold 起征金额 默认值为5000
     * @return bool|float|int float | false 返回值为应缴税金额 参数错误时返回false
     */
    public static function getPersonalIncomeTax($salary, $deduction = 10, $threshold = 5000)
    {
        if (!is_numeric($salary) || !is_numeric($deduction) || !is_numeric($threshold)) {
            return false;
        }

        if ($salary <= $threshold) {
            return 0;
        }

        $levels = [3000, 12000, 25000, 35000, 55000, 80000, PHP_INT_MAX];
        $rates = [0.03, 0.1, 0.2, 0.25, 0.3, 0.35, 0.45];
        $taxableIncome = $salary - $threshold - $deduction;
        $tax = 0;
        foreach ($levels as $k => $level) {
            $previousLevel = isset($levels[$k - 1]) ? $levels[$k - 1] : 0;
            if ($taxableIncome <= $level) {
                $tax += ($taxableIncome - $previousLevel) * $rates[$k];
                break;
            }
            $tax += ($level - $previousLevel) * $rates[$k];
        }
        $tax = round($tax, 2);
        return $tax;
    }