PHP 计算个人所得税

前言

使用此种计算方式前，需要知道当年已纳税总额度以及本月应纳税额度。这两个参数需要自行根据各自系统数据计算出来，此方法只实现个税计算算法。此外，需要打开PHP的bc扩展。

参数

方法传入两个参数即可，当月应纳税额度以及本年已纳税总额度（不含当月）。

• $lauwen_monthly_tax_amount：当月应纳税额度。
• $lauwen_taxed_amount：当年已纳税总额度（不含当月）。

实现

首先是确认两个层级，一个是本年已纳税总额度所在税率层级（不含当月），另一个是本月之后本年已纳税总额度所在税率层级（含当月）；最后在计算个税时使用了两种计算方式，一种是分别计算在每个税率层级的税额；另一种是使用速算扣除数进行计算，按需使用就好。

分级计算核心代码

    // 分级法计算
    $tax = '0';
    while ($new_key >= $old_key) {
        $level_amount = bcsub($lauwen_month_after_amount, $lauwen_tax_amounts[$new_key], 6);
        if ($new_key == $old_key) {
            $level_amount = bcsub($lauwen_month_after_amount, $lauwen_taxed_amount, 6);
        }

        $tax = bcadd($tax, bcmul($level_amount, $lauwen_tax_rates[$new_key]), 2);

        $lauwen_month_after_amount = $lauwen_tax_amounts[$new_key];
        $new_key --;
    }

速算扣除计算核心代码

    // 速算扣除法计算
    $quickly_old_tax = bcsub(bcmul($lauwen_taxed_amount, $lauwen_tax_rates[$old_key], 6), $lauwen_tax_quickly[$old_key], 6);
    $quickly_new_tax = bcsub(bcmul($lauwen_month_after_amount, $lauwen_tax_rates[$new_key], 6), $lauwen_tax_quickly[$new_key], 6);
    $tax0 = bcsub($quickly_new_tax, $quickly_old_tax, 2);

全部代码

function personTax($lauwen_monthly_tax_amount, $lauwen_taxed_amount) {
    $lauwen_tax_amounts = ['0', '36000', '144000', '300000', '420000', '660000', '960000'];
    $lauwen_tax_rates = ['0.03', '0.1', '0.2', '0.25', '0.3', '0.35', '0.45'];
    $lauwen_tax_quickly = ['0', '2520', '16920', '31920', '52920', '85920', '181920'];

    $lauwen_taxed_amount = (string)$lauwen_taxed_amount;
    $lauwen_monthly_tax_amount = (string)$lauwen_monthly_tax_amount;
    $lauwen_month_after_amount = bcadd($lauwen_taxed_amount, $lauwen_monthly_tax_amount, 6);

    // 确定当月前当年已纳税总额度所属级别，以及当月之后当年已纳税总额度所属级别
    $old_key = 0;           // 当月前级别
    $new_key = 0;           // 当月后级别
    $end_key = count($lauwen_tax_amounts) - 1;
    foreach ($lauwen_tax_amounts as $key => $val) {
        if ($key == $end_key) {             // 最后一级
            if (bccomp($lauwen_taxed_amount, $val, 2) == 1) {
                $old_key = $key;
            }

            if (bccomp($lauwen_month_after_amount, $val, 2) == 1) {
                $new_key = $key;
            }
        } else {            //
            if (bccomp($lauwen_taxed_amount, $val, 2) == 1 && bccomp($lauwen_taxed_amount, $lauwen_tax_amounts[$key+1], 2) == -1) {
                $old_key = $key;
            }

            if (bccomp($lauwen_month_after_amount, $val, 2) == 1 && bccomp($lauwen_month_after_amount, $lauwen_tax_amounts[$key+1], 2) == -1) {
                $new_key = $key;
                break;
            }
        }
    }

    // 速算扣除法计算
    $quickly_old_tax = bcsub(bcmul($lauwen_taxed_amount, $lauwen_tax_rates[$old_key], 6), $lauwen_tax_quickly[$old_key], 6);
    $quickly_new_tax = bcsub(bcmul($lauwen_month_after_amount, $lauwen_tax_rates[$new_key], 6), $lauwen_tax_quickly[$new_key], 6);
    $tax0 = bcsub($quickly_new_tax, $quickly_old_tax, 2);

    // 差额法计算
    $tax = '0';
    while ($new_key >= $old_key) {
        $level_amount = bcsub($lauwen_month_after_amount, $lauwen_tax_amounts[$new_key], 6);
        if ($new_key == $old_key) {
            $level_amount = bcsub($lauwen_month_after_amount, $lauwen_taxed_amount, 6);
        }

        $tax = bcadd($tax, bcmul($level_amount, $lauwen_tax_rates[$new_key]), 2);

        $lauwen_month_after_amount = $lauwen_tax_amounts[$new_key];
        $new_key --;
    }

   return json_encode([
       "normal"     =>  $tax,
       "quickly"    =>  $tax0,
   ]);
}