Comment mettre en œuvre un système de commande en ligne simple en utilisant PHP

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "orders";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接是否成功
if ($conn->connect_error) {
  die("Connection failed: " . $conn->connect_error);
}

function get_menu($conn) {
  $sql = "SELECT * FROM menu";
  $result = $conn->query($sql);

  if ($result->num_rows > 0) {
    $menu = array();

    // 将结果转换为数组
    while($row = $result->fetch_assoc()) {
      array_push($menu, $row);
    }

    return json_encode($menu);
  } else {
    return "0 结果";
  }
}

function add_to_cart($id, $quantity) {
  // 检查物品是否已存在于购物车中
  if(isset($_SESSION["cart"][$id])) {
    $_SESSION["cart"][$id] += $quantity;
  } else {
    $_SESSION["cart"][$id] = $quantity;
  }
}

function display_cart() {
  $output = "";

  if(!empty($_SESSION["cart"])) {
    $total = 0;

    $output .= "<table>";
    $output .= "<tr><th>物品</th><th>数量</th><th>价格</th><th>总价</th></tr>";

    // 遍历购物车中的每个物品
    foreach($_SESSION["cart"] as $id => $quantity) {
      $sql = "SELECT name, price FROM menu WHERE id = $id";
      $result = $conn->query($sql);

      if ($result->num_rows > 0) {
        $row = $result->fetch_assoc();

        $name = $row["name"];
        $price = $row["price"];
        $total_price = $price * $quantity;
        $total += $total_price;

        $output .= "<tr><td>$name</td><td>$quantity</td><td>$price 元</td><td>$total_price 元</td></tr>";
      }
    }

    $output .= "<tr><td colspan='3'>总价</td><td>$total 元</td></tr>";
    $output .= "</table>";
  } else {
    $output .= "购物车为空";
  }

  return $output;
}

function submit_order($name, $address) {
  $items = json_encode($_SESSION["cart"]);

  // 将订单插入到数据库
  $sql = "INSERT INTO orders (name, address, items)
  VALUES ('$name', '$address', '$items')";

  if ($conn->query($sql) === TRUE) {
    // 清空购物车
    $_SESSION["cart"] = array();

    return true;
  } else {
    return false;
  }
}