How to choose which is the key of the PHP array and not that pattern (0 => ... 1 => ... 2 =>... )

Question

The thing is, I’m making a small system where the person can include in an array the desired products, he clicks on the "Insert" product button and some information is collected from that product and stored in an array, in this case, the "id_product", "product name", "product preco_tag", "quantity_product" is the information collected and saved and that goes on, the person will insert products and they will be stored in an array. I have the following structure in code:

case 'add':

    $produto = new ProdutoController();
    $retorno = $produto->selectId($_GET['id']);

    if (count($retorno) > 0)
    {
        foreach($retorno as $dados => $value)
        {
            $arrayProduto[$retorno[$dados]->id_produto] = array(
                                                    'id_prod' => $retorno[$dados]->id_produto, 
                                                    'nome_prod' => $retorno[$dados]->nome_produto,
                                                    'code_prod' => $retorno[$dados]->code_produto,
                                                    'preco_prod' => $retorno[$dados]->preco_produto,
                                                    'qtd_prod' => $_POST['txt_quantity']);

        }

        if(!empty($_SESSION['cart_item'])) 
        {    
            foreach ($arrayProduto as $key => $value) 
            {
                if(array_key_exists($key, $_SESSION['cart_item'])) 
                {    
                    ?> 
                        <script language="JavaScript" type="text/javascript"> 
                        alert ("Este produto já foi inserido!") 
                        </script>
                    <?php
                        header("Location: /view/cart/cart.php");
                }
                else
                {
                    $_SESSION['cart_item'] = array_merge($_SESSION['cart_item'], $arrayProduto);
                    header("Location: /view/cart/cart.php");
                }
            }
        }
        else
        {
            $_SESSION['cart_item'] = $arrayProduto;
            header("Location: /view/cart/cart.php");
        }
    }
    else
    {
        header("Location: /view");
    }
    break;

In this case, I have the following doubts: How do I choose which will be the key of the array? If I leave it the way it is, it will insert so

    array (size=3)
    0 => 
    array (size=5)
          'id_prod' => string '1' (length=1)
          'nome_prod' => string 'GTX 1050 Gigabyte 2GB 128bits' (length=29)
          'code_prod' => string 'ab01' (length=4)
          'preco_prod' => string '599.90' (length=6)
          'qtd_prod' => string '1' (length=1)
    1 =>
    array (size=5)
          'id_prod' => string '5' (length=1)
          'nome_prod' => string 'VGA GTX 1070 Gigabyte 6GB 256bits' (length=33)
          'code_prod' => string 'ab07' (length=4)
          'preco_prod' => string '1399.90' (length=7)
          'qtd_prod' => string '1' (length=1)
      2 => 
        array (size=5)
          'id_prod' => string '2' (length=1)
          'nome_prod' => string 'VGA GTX 1050Ti EVGA 4GB 128bits' (length=31)
          'code_prod' => string 'ab04' (length=4)
          'preco_prod' => string '699.90' (length=6)
          'qtd_prod' => string '1' (length=1)

However I do not want to be these default numbers (0, 1, 2, 3...), I want to be the "id_product" the key of the array, for later when I delete a product, I take the "id_prod", compare with the key of the array and where it is equal I give "unset", ie, I will be removing. I need to make the array key equal to the id_product, how do I do this? Thank you!