Why isn't working my notifications script with sales module?

Question

The sales notifications is not working because this line amountArray[$idproduct] += $amount; is returning me offset. I don't know how to fix it.

My full function is this:

function saveAllSaleDetails($idsale, $sale) {
    $this->conexion->startTransaction();
    $amountArray = [];
    try {
        foreach ($sale as $detail):
            $idproduct = $detail['id'];
            $amount = $detail['amount'];
            $price = $detail['price'];
            $subtotal = $detail['subtotal'];
            $iduser = 1;
            $this->saveSaleDetail($idsale, $idproduct, $amount, $price, $subtotal, $iduser);
            $amountArray[$idproduct] += $amount;
            $stock = $this->product->getProductStock($idproduct);
            $stock = $stock[0][0] - $amountArray[$idproduct];

            if ($stock <= 20) {
                $product = $this->product->getProductById($idproduct);
                $message = $product[0][1]." stock is bellow 20.";
                notification::add($message, $idproduct, 'warning', 'product.php');
            }
        endforeach;

        $this->conexion->commit();
        $this->conexion->cerrar();
        return true;

    } catch (Exception $e) {
        $this->conexion->rollback();
        $this->conexion->cerrar();
        var_dump($e->getMessage());
        return false;
    }
}

Answer 1

The problem is because of this line,

$amountArray[$idproduct] += $amount;

The above statement can be expanded as

$amountArray[$idproduct] = $amountArray[$idproduct] + $amount;

Initially during the first iteration of foreach loop, $amountArray[1] and $amountArray[2] are not set, and hence you're getting these undefined offset error.

So instead of

$amountArray[$idproduct] += $amount;

Do this:

$amountArray[$idproduct] = isset($amountArray[$idproduct]) ? $amountArray[$idproduct] + $amount : $amount;