app\models\Invoice::calculateTax PHP Method

    private function calculateTax(&$taxes, $name, $rate, $amount, $paid)
    {
        if (!$amount) {
            return;
        }
        $amount = round($amount, 2);
        $paid = round($paid, 2);
        $key = $rate . ' ' . $name;
        if (!isset($taxes[$key])) {
            $taxes[$key] = ['name' => $name, 'rate' => $rate + 0, 'amount' => 0, 'paid' => 0];
        }
        $taxes[$key]['amount'] += $amount;
        $taxes[$key]['paid'] += $paid;
    }