I'm completely new to ajax so I'm not sure if I'm doing it correctly. I'm trying to pass an Javascript array to a PHP file. And if succesfully open the PHP file and display the array. The error I'm getting after opening the file is:

undefined $productList

Meanwhile if I inspect and look into the network tab the ajax is sent successfully and the response is also fine (I get to see the contents of the array), but when I try to open the file it throws me errors. So my question is how do I open a file where I send my data to with ajax?
Array:

array(6) { ["name"]=> string(28) "Inuyasha by rumiko takahashi" ["image"]=> string(35) "https://via.placeholder.com/195x280" ["id"]=> string(1) "5" ["count"]=> string(1) "1" ["price"]=> string(2) "12" ["basePrice"]=> string(2) "12" }

The ajax:

function passArray(){
    $.ajax({
        type: "POST",
        url: "assets/js/test.php",
        data: {productsInCart:productsInCart},
        error: function(){
          alert('something went wrong..');
        }
    });
    //window.open("assets/js/test.php", "_self"); //how to open the file where the data is sent?
}

The PHP code:

if($_POST){
    $productlist = $_POST;
}else{
    $productlist = "";
    echo "doesnt work ";
    
    die();
}
//var_dump($productlist);


foreach($productlist as $number){
    foreach ($number as $cat => $prop) {
        echo $cat;
        echo $prop;
    }
}

$html .= "</table>";