下面是一个用于计算个税专项扣除附加的函数，主要的功能是根据税前收入和税前扣除额来计算个税专项扣除附加。

具体地，该函数首先通过计算税前收入和税前扣除额之差来确定纳税起征点。接下来，根据纳税起征点和个人缴税状况，在个税专项扣除附加的规定基础上，计算出个税专项扣除附加。

最后，该函数会返回计算出的个税专项扣除附加金额。

function calculateTaxDeduction($income, $deductions) {
    $taxableIncome = $income - $deductions;
    $taxRate = 0.3;
    $quickDeduction = 0;
    if ($taxableIncome <= 36000) {
        $taxRate = 0.03;
        $quickDeduction = 0;
    } else if ($taxableIncome <= 144000) {
        $taxRate = 0.1;
        $quickDeduction = 2520;
    } else if ($taxableIncome <= 300000) {
        $taxRate = 0.2;
        $quickDeduction = 16920;
    } else if ($taxableIncome <= 420000) {
        $taxRate = 0.25;
        $quickDeduction = 31920;
    } else if ($taxableIncome <= 660000) {
        $taxRate = 0.3;
        $quickDeduction = 52920;
    } else if ($taxableIncome <= 960000) {
        $taxRate = 0.35;
        $quickDeduction = 85920;
    } else {
        $taxRate = 0.45;
        $quickDeduction = 181920;
    }
    $tax = $taxableIncome * $taxRate - $quickDeduction;
    return $tax;
}

$income = 100000;
$deductions = 5000;
$tax = calculateTaxDeduction($income, $deductions);
echo "您的所得税为：" . $tax . " 元";

下面的函数用于计算起征点和专项扣除。

<?php
// 定义基本扣除数额
$basic_deduction = 5000;

// 计算专项扣除
function getDeductions($income, $deductions_list) {
    global $basic_deduction;

    // 减去基本扣除数额
    $taxable_income = $income - $basic_deduction;

    // 遍历专项扣除列表
    $total_deductions = 0;
    foreach ($deductions_list as $deduction) {
        if ($taxable_income > 0) {
            $total_deductions += min($taxable_income, $deduction['amount']);
            $taxable_income -= $deduction['amount'];
        } else {
            break;
        }
    }

    return $total_deductions;
}

// 定义专项扣除列表
$deductions_list = [
    ['name' => '住房贷款利息', 'amount' => 1500],
    ['name' => '住房租金', 'amount' => 1000],
    ['name' => '子女教育', 'amount' => 1000],
    ['name' => '继续教育', 'amount' => 2000],
    ['name' => '赡养老人', 'amount' => 1500],
    ['name' => '大病医疗', 'amount' => 4000],
];

// 计算收入为10000元的专项扣除额度
$income = 10000;
$deductions = getDeductions($income, $deductions_list);

echo "收入为{$income}元，专项扣除额度为{$deductions}元";

请注意，以上代码仅为示例，实际扣除额度应根据个人情况和最新的政策法规计算。

由于不同城市的住房租金扣除金额不同，下面给出函数，根据给定的城市名称，计算住房租金扣除。

function calculateRentDeduction($city, $rentPaid) {
    // 设置各个城市的住房租金比例
    $rentPercentage = array(
        'Beijing' => 0.3,
        'Shanghai' => 0.35,
        'Guangzhou' => 0.4,
        'Shenzhen' => 0.45,
        'Other' => 0.25
    );

    // 如果该城市有对应的住房租金比例，则计算住房租金扣除额
    if (isset($rentPercentage[$city])) {
        $deduction = $rentPaid * $rentPercentage[$city];
    } else {
        $deduction = $rentPaid * $rentPercentage['Other'];
    }

    return $deduction;
}

该函数接收两个参数：$city 和 $rentPaid。函数内部定义了一个数组 $rentPercentage，用于存储各个城市的住房租金扣除比例。如果给定的城市名称在数组中有对应的住房租金比例，则使用该比例计算住房租金扣除额；否则，使用默认的比例，即 Other