如何增加同一产品的数量，以及如何减少数量？

Question

The problem that I encountered was it it cannot increase while I clicking the adding button for the second time, and I don't have the idea of how to do the decrease button.

if (isset($_POST["add"])){
if (isset($_SESSION["cart"])){
    $item_array_id = array_column($_SESSION["cart"], "id");
    if(!in_array($_GET["id"], $item_array_id)){
        $count = count($_SESSION["cart"]);
        $item_array = array(
            'id' => $_GET["id"],
            'item_name' => $_POST["hidden_name"],
            'product_price' => $_POST["hidden_price"],
            'item_quantity' => $_POST["quantity"],
        );
        $_SESSION["cart"][$count] = $item_array;
        echo '<script>window.location="counter.php"</script>';
    }else{
        echo '<script>alert("Product is already Added to List")</script>';
        echo '<script>window.location="counter.php"</script>';
    }
}else{
    $item_array = array(
        'id' => $_GET["id"],
        'item_name' => $_POST["hidden_name"],
        'product_price' => $_POST["hidden_price"],
        'item_quantity' => $_POST["quantity"],
    );
    $_SESSION["cart"][0] = $item_array;
}

}

<?php
        $conn = mysqli_connect("localhost", "root", "", "test");
        $query = "SELECT product_id, product_name, product_price, image FROM product ORDER BY product_id ASC ";
        $result = mysqli_query($conn,$query);
        if(mysqli_num_rows($result) > 0) {

            while ($row = mysqli_fetch_array($result)) {

                ?>
                <div class="col-md-3">

                    <form method="post" action="counter.php?action=add&id=<?php echo $row["product_id"]; ?>">

                        <div class="product">
                            <img src="img/<?php echo $row["image"]; ?>" style="width:100px; height:100px">
                            <h5 class="text-info"><?php echo $row["product_name"]; ?></h5>
                            <h5 class="text-danger"><?php echo "RM " . $row["product_price"]; ?></h5>
                            <input type="text" name="quantity" class="form-control" value="1">
                            <input type="hidden" name="hidden_name" value="<?php echo $row["product_name"]; ?>">
                            <input type="hidden" name="hidden_price" value="<?php echo $row["product_price"]; ?>">
                            <input type="submit" name="add" style="margin-top: 5px;" class="btn btn-success" value="+">
                            <input type="submit" name="minus" style="margin-top: 5px;" class="btn btn-success" value="-">
                        </div>
                    </form>
                </div>
                <?php
            }
        }
    ?>

<div style="clear: both"></div>
    <div class="table-responsive">
        <table class="table table-bordered">
        <tr>
            <th width="30%">Product Name</th>
            <th width="10%">Quantity</th>
            <th width="10%">Total Price</th>
            <th width="17%">Remove Item</th>
        </tr>

        <?php
            if(!empty($_SESSION["cart"])){
                $total = 0;
                foreach ($_SESSION["cart"] as $key => $value) {
                    ?>
                    <tr>
                        <td><?php echo $value["item_name"]; ?></td>
                        <td><?php echo $value["item_quantity"]; ?> <a href="counter.php?action1=subtract&id=<?php echo $value["product_id"]; ?>"><input type="submit" name="subtract" style="width:20px; height:25px" class="btn btn-success" 
                                value="-"></td>
                        <td>RM <?php echo number_format($value["item_quantity"] * $value["product_price"], 2); ?></td>
                        <td style="text-align:center;"><a href="counter.php?action=delete&id=<?php echo $value["product_id"]; ?>"><img src="img/icon-delete.png"/></a></td>

                    </tr>
                    <?php
                    $total = $total + ($value["item_quantity"] * $value["product_price"]);
                }
                    ?>
                    <tr>
                        <td colspan="2" align="right">Total</td>
                        <th align="right">RM <?php echo number_format($total, 2); ?></th>
                        <td></td>
                    </tr>
                    <?php
                }
            ?>
        </table>

    </div>

The output of adding only shows for the first clicked, but it will be displayed "Product is already Added to List" for the second clicked, cannot increase the product quantity. The actual result should be increasing quantity. Same question for the decrease product quantity

Answer 1

You have to change the logic of increment and decrement the quantity.

If you press add button then increase the value of product by 1 and same vice versa. So first of all your need to get the current quantity of the added product and then do the operator add or delete.

$_SESSION["cart"][$count]['item_quantity']=$_SESSION["cart"][$count]['item_quantity'] + $_POST["quantity"];  // For add

Basic idea given below.

if (isset($_POST["add"])){
   if (isset($_SESSION["cart"])){
      $item_array_id = array_column($_SESSION["cart"], "id");
       if(!in_array($_GET["id"], $item_array_id)){
          $count = count($_SESSION["cart"]);
          $_SESSION["cart"][$count]['item_quantity']=$_SESSION["cart"][$count]['item_quantity']+$_POST["quantity"];
           $item_array = array(
                'id' => $_GET["id"],
                'item_name' => $_POST["hidden_name"],
                'product_price' => $_POST["hidden_price"],
                'item_quantity' => $_POST["quantity"],
           );
           $_SESSION["cart"][$count] = $item_array;
          echo '<script>window.location="counter.php"</script>';
      }else{
          echo '<script>alert("Product is already Added to List")</script>';
          echo '<script>window.location="counter.php"</script>';
      }
}else{
     $_SESSION["cart"][0]['item_quantity']=$_SESSION["cart"][0]['item_quantity']- $_POST["quantity"];
    $item_array = array(
        'id' => $_GET["id"],
        'item_name' => $_POST["hidden_name"],
        'product_price' => $_POST["hidden_price"],
        'item_quantity' => $_POST["quantity"],
    );
    $_SESSION["cart"][0] = $item_array;
 }
}

Answer 2

You need to take the quantity of the previous addition and add or subtract them together. You can also use $_SESSION["cart"] and store the ID as the index.

You should be able to use: (this isn't tested)

if (isset($_POST["add"])){
    if(!isset($_SESSION["cart"][$_GET["id"]]['item_quantity'])){$_SESSION["cart"][$_GET["id"]]['item_quantity'] = 0;}
    $_SESSION["cart"][$_GET["id"]]['item_name'] = $_POST["hidden_name"];
    $_SESSION["cart"][$_GET["id"]]['product_price'] = $_POST["hidden_price"];
    $_SESSION["cart"][$_GET["id"]]['item_quantity'] = $_SESSION["cart"][$_GET["id"]]['item_quantity'] + $_POST["quantity"];

    header('Location: counter.php');
}
if (isset($_POST["decrease"])){
    if(!isset($_SESSION["cart"][$_GET["id"]]['item_quantity'])){$_SESSION["cart"][$_GET["id"]]['item_quantity'] = 0;}
    $_SESSION["cart"][$_GET["id"]]['item_name'] = $_POST["hidden_name"];
    $_SESSION["cart"][$_GET["id"]]['product_price'] = $_POST["hidden_price"];
    $_SESSION["cart"][$_GET["id"]]['item_quantity'] = ( ( ( $_SESSION["cart"][$_GET["id"]]['item_quantity'] - $_POST["quantity"] ) > 0  ) ? $_SESSION["cart"][$_GET["id"]]['item_quantity'] - $_POST["quantity"] : 0 );

    header('Location: counter.php');
}