PHP/MySQL：更新 int where id = column？

Question

I want to list some products out of an SQL-DB, which works perfectly. Now the user should click on two different buttons to add +1 to the amount of this specific product and subtract -1 of this.

This is how my front-end looks like:

<?php
    require_once('../system/config.php');
    require_once('../system/server.php');

// Create connection
$conn = new mysqli($db_host, $db_user, $db_pass, $db_name);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}
$sql = "SELECT item, count, date FROM shop";
$result = $conn->query($sql);

?> 

[...]

<form>
    <table>
        <form method="post" action="frontend.php">
            <tr>
                <th></th>
                <th></th>
                <th>Amount</th>
                <th>Item</th>
                <th>Date</th>
                <th></th>
            </tr>
            <?php while($row = mysqli_fetch_array($result)):?>
            <tr>
                <td><button class="delete" name="delete">x</button></td>
                <td><button class="minus" name="minus">-</button></td>
                <td><?php echo $row['count'];?></td>
                <td><?php echo $row['item'];?></td>
                <td><?php echo $row['date'];?></td>
                <td><button class="plus" name="plus">+</button></td> 
            </tr>
        </form>
        <?php endwhile;?>
    </table>
</form>

Here works everything so far, it lists the datas out of the DB.

For my backend-code I thought I will work with 'UPDATE'. I post only one function, the two others are quiet similar.

$db = mysqli_connect('xxx', 'xxx', 'xxx', 'xx');    

//Item add
if (isset($_POST['plus'])) {

    $count = mysqli_real_escape_string($db, $_POST['count']);

    $query = "UPDATE `shop` SET `count` = `count` + 1 WHERE `id` = '51'";
    mysqli_query($db, $query) or die(mysqli_error($db));
    header('location: frontend.php');
};

It works if I give a specific ID-number. What can I do if I want the ID who should be changed is the ID the column where the button is clicked?

Answer 1

What really should be done is:

<!-- NOTE: no <form> tags around and inside <table> -->
<table>
    <tr>
        <th></th>
        <th></th>
        <th>Amount</th>
        <th>Item</th>
        <th>Date</th>
        <th></th>
    </tr>
    <?php while($row = mysqli_fetch_array($result)):?>
    <tr>
        <td>
            <!-- form tag appears here -->
            <form method="post" action="frontend.php">
                <button type="submit" class="delete" name="delete">x</button>
                <input type="hidden" name="item_id" value="<?php echo $row['id'];?>" />
            </form>
        </td>
        <td>
            <!-- form tag appears here -->
            <form method="post" action="frontend.php">
                <button type="submit" class="minus" name="minus">-</button>
                <input type="hidden" name="item_id" value="<?php echo $row['id'];?>" />
            </form>
        </td>
        <td><?php echo $row['count'];?></td>
        <td><?php echo $row['item'];?></td>
        <td><?php echo $row['date'];?></td>
        <td>
            <!-- form tag appears here -->
            <form method="post" action="frontend.php">
                <button type="submit" class="plus" name="plus">+</button>
                <!-- also `input` with type hidden appears, which holds 
                 the ID of current value (I assume it is `id` column) -->
                <input type="hidden" name="item_id" value="<?php echo $row['id'];?>" />
            </form>
        </td> 
    </tr>
    <?php endwhile;?>

On server side:

if (isset($_POST['plus'])) {

    // you don't need $count
    //$count = mysqli_real_escape_string($db, $_POST['count']);

    $query = "UPDATE `shop` SET `count` = `count` + 1 WHERE `id` = ?";
    // As we receive data from user input, it should be considered 
    // not safe that's why we use prepared statements
    $stmt = $db->prepare($query);
    $stmt->bind_param('s', $_POST['item_id']);
    $stmt->execute();

    header('location: frontend.php');
};
// similar code can be used to `delete`/`minus` actions