将数组添加到存储在cookie中的数组

having a problem with storing an array variable as a cookie, then adding in separate arrays into the cookie already containing the array

$mysqli = mysqli_connect($db_host,$db_user,$db_pass,$db_base);
        if (mysqli_connect_errno()) 
        {
            printf("Connect failed: %s
", mysqli_connect_error());
            exit();
        }
        // Displays a message saying product was added to the basket
        $message = $_POST['product_name']." was added to the basket";
        echo "<script>alert(".$message.")</script>";

        // Sets the basket
        $itemID = $_POST['product_id'];
        $itemQuantity = $_POST['product_quantity'];
        if ($itemQuantity > 0)
        {
            $items = [$itemID, $itemQuantity];

            // Returns cookie value as array
            $basket_array = (unserialize($_COOKIE['eg_basket']));

            // Adds to array into cookie array
            $basket = serialize(array_push($basket_array, $items));

            // Sets basket back as cookie
            setcookie('eg_basket', $basket); // will expire on browser close

            // Displays message
            echo "<h3 style='text-align:center'>".$_POST['product_name']." was added to the basket</h3>";
            echo "<br/>";
            echo "<p style='text-align:center'>Please click below to return to the previous page</p>";
        }
        else
        {
            echo "<h3 style='text-align:center'>ERROR: ".$_POST['product_name']." was not added to the basket, invalid quantity given</h3>";
        }

            echo "<form method='POST' action='product_info.php'><input style='display:none' type='number' name='product_id_POST' value='".$_POST['product_id']."'><input style='text-align:center' type='submit' value='Return'></form>";


        // close the connection
        mysqli_close($mysqli);
    ?>

What i am tryng to do is to create a cookie that will store arrays of product id;s and the quantity

e.g. cookie = [[product_id, quantity],[product_id, quantity],.......]; however

im pretty sure this is not the case, but this is the code i am using to create the basket cookie if it does not exist (could this be the reason why the cookie will not accept any new values

// Checks is cookie is already set - basket only
    if (!isSet($_COOKIE['eg_basket']))
    {
        $basket = serialize([]);
        setcookie('eg_basket', $basket); // will expire on browser close
    }

Thanks, any help is greatly appreciated

Bull

You cannot send a cookie to the browser when the headers already have been sent, see the manual.

So you need to log your message without echoing anything here:

echo "<script>alert(".$message.")</script>";

And make sure no other output is sent to the browser before your setcookie() line.