Return Tax base on Salary

CaculateTaxBySalary.php

<?php
class TaxSalary
{
    // Input 1000000.
    function salary($gross_local)
    {

        if (!is_numeric($gross_local) && floatval($gross_local)) {
             die ('Please input is must type Numeric.');
        }

        if ($gross_local < 0) {
            die ('Number must be greater than 0.');
        }

        if (($gross_local > 0) && ($gross_local <= 5000000)){
            $tax = 0;
        }elseif (($gross_local > 5000000) && ($gross_local <= 15000000)){
            $tax = ($gross_local - 5000000) * 0.1;
        }elseif (($gross_local > 15000000) && ($gross_local <= 25000000)){
            $tax = 1000000 + (($gross_local - 15000000) * 0.2);
        }elseif (($gross_local > 25000000) && ($gross_local <= 40000000)){
            $tax = 3000000 + (($gross_local - 25000000) * 0.3);
        }elseif ($gross_local > 40000000){
            $tax = 7500000 + (($gross_local - 40000000) * 0.4);
        }
        return $tax;
    }
}

$objTax = new TaxSalary();
$tax = $objTax->salary(15000000);
print 'Số tiền phải đóng thuế là '.number_format($tax);