How to change the value of an Array inside a $

Question

I don’t have much idea how to use array, let alone involve php in it, so I took this code from the internet and managed to make some changes that I found necessary to fit well into the site I’m developing for a school project. I changed a lot of things with the exception of this part of the code, in which she adds items to a cart (Does it perfectly) but when I try to add the same item a second time, instead of it adding up the quantities it just issues an Alert saying that the item has already been added.

I’ve seen many tutorials on how to change the values of an array, but this one specifically is within a Session, variable $ and other things that I’ve learned a little bit.

Can help?

if (isset($_POST["add_to_cart"])) {
        if (isset($_SESSION["shopping_cart"])) {
            $item_array_id = array_column($_SESSION["shopping_cart"], "item_id");
            if (!in_array($_GET["id"], $item_array_id)) {
                $count      = count($_SESSION["shopping_cart"]);
                $item_array = [
                    'item_id'       => $_GET["id"],
                    'item_name'     => $_POST["hidden_name"],
                    'item_price'    => $_POST["hidden_price"],
                    'item_quantity' => $_POST["quantity"],
                ];
                $_SESSION["shopping_cart"][$count] = $item_array;
            } else {
                echo '<script>alert("Item ja adicionado")</script>';
                echo '<script>window.location="foodlist.php"</script>';
            }

Edit1: I don’t know if it’s necessary, but on the same page you have more this array code.

    else
      {
        $item_array                   = array(
            'item_id' => $_GET["id"],
            'item_name' => $_POST["hidden_name"],
            'item_price' => $_POST["hidden_price"],
            'item_quantity' => $_POST["quantity"]
        );
        $_SESSION["shopping_cart"][0] = $item_array;
      }
  }
if (isset($_GET["action"]))
  {
    if ($_GET["action"] == "delete")
      {
        foreach ($_SESSION["shopping_cart"] as $keys => $values)
          {
            if ($values["item_id"] == $_GET["id"])
              {
                unset($_SESSION["shopping_cart"][$keys]);
                echo '<script>window.location="foodlist.php"</script>';
              }
          }
      }

  }

Answer 1

if (isset($_POST["add_to_cart"])) {
    if (isset($_SESSION["shopping_cart"])) {
        $item_array_id = array_column($_SESSION["shopping_cart"], "item_id");
        if (!in_array($_GET["id"], $item_array_id)) {
            $count      = count($_SESSION["shopping_cart"]);
            $item_array = [
                'item_id'       => $_GET["id"],
                'item_name'     => $_POST["hidden_name"],
                'item_price'    => $_POST["hidden_price"],
                'item_quantity' => $_POST["quantity"],
            ];
            $_SESSION["shopping_cart"][$count] = $item_array;
        } else {
            //echo '<script>alert("Item ja adicionado")</script>';
            //echo '<script>window.location="foodlist.php"</script>';
            $key = array_search($_GET["id"], $item_array_id);
            $_SESSION["shopping_cart"][$key]["item_quantity"] += $_POST["quantity"];
        }