public static function calculateSalary($salary)
 {
     $liquidSalary = 0;
     $inss = 0;
     $irpf = 0;
     $aliquots = new SALARYServices();
     $inss = $aliquots->calculateInss($salary);
     // Salario descontado o inss
     $desInss = $salary - $inss;
     //Calculando a alíquota do imposto de renda passando o salario com desconto do inss
     $irpf = $aliquots->calculateIrpf($desInss);
     //calculando o salário liquido;
     $liquidSalary = $salary - $inss - $irpf;
     return $aliquots->result($liquidSalary, $inss, $irpf);
 }
<?php

require 'salaryServices.php';
$salary = $_GET['salary'];
$result = 0;
if (is_numeric($salary)) {
    $result = SALARYServices::calculateSalary($salary);
}
echo json_encode($result);