How to avoid nested if-else statements

When developing API methods to return JSON with details I often find myself creating the following code:

    $response = array('success' => false);

    $user = User::getSessionUser();
    if($user->id > 0){
        if(ItemSKU::stringIsValid($sku)){

            $itemSKU = ItemSKU::getFromString($sku);
            if($itemSKU->isAvailable()){
                if(Carts::add($user->id, $sku)){
                    $response['success'] = true;
                }
                else{
                    $response['error'] = "Item was already added";
                }
            }
            else{
                $response['error'] = "Unavailable Item";
            }
        }
        else{
            $response['error'] = "Invalid SKU";
        }
    }
    else{
        $response['error'] = "Invalid User";
    }

    return response()->json($response);

How can I avoid using this nested if-else chain? Thanks!