app\models\Invoice::getTaxable PHP Method

    public function getTaxable()
    {
        $total = 0;
        foreach ($this->invoice_items as $invoiceItem) {
            $total += $invoiceItem->qty * $invoiceItem->cost;
        }
        if ($this->discount > 0) {
            if ($this->is_amount_discount) {
                $total -= $this->discount;
            } else {
                $total *= (100 - $this->discount) / 100;
                $total = round($total, 2);
            }
        }
        if ($this->custom_value1 && $this->custom_taxes1) {
            $total += $this->custom_value1;
        }
        if ($this->custom_value2 && $this->custom_taxes2) {
            $total += $this->custom_value2;
        }
        return $total;
    }