如何在PHP中实现在线二手交易平台？

<?php
// 连接数据库
$con = mysqli_connect("localhost", "username", "passWord", "database_name");

// 添加用户
$name = $_POST["name"];
$email = $_POST["email"];
$password = $_POST["password"];
$sql = "INSERT INTO users (name, email, password) VALUES('$name', '$email', '$password')";
mysqli_query($con, $sql);
?>

<?php
// 添加商品
$name = $_POST["name"];
$description = $_POST["description"];
$price = $_POST["price"];
$sql = "INSERT INTO products (name, description, price) VALUES('$name', '$description', '$price')";
mysqli_query($con, $sql);
?>

<?php
// 连接数据库
$con = mysqli_connect("localhost", "username", "password", "database_name");

// 搜索并显示商品
$search = $_GET["search"];
$sql = "SELECT * FROM products WHERE name LIKE '%$search%'";
$result = mysqli_query($con, $sql);
while ($row = mysqli_fetch_assoc($result)) {
    echo $row["name"] . "<br>";
}
?>

<?php
// 连接数据库
$con = mysqli_connect("localhost", "username", "password", "database_name");

// 添加订单并更新库存
$product_id = $_GET["id"];
$user_id = $_SESSION["user_id"];
$sql = "INSERT INTO orders (product_id, user_id, status) VALUES('$product_id', '$user_id', 'pending')";
mysqli_query($con, $sql);

$sql = "UPDATE products SET stock = stock - 1 WHERE id = '$product_id'";
mysqli_query($con, $sql);
?>

<?php
// 添加评价
$product_id = $_POST["product_id"];
$user_id = $_POST["user_id"];
$rating = $_POST["rating"];
$comment = $_POST["comment"];
$sql = "INSERT INTO feedback (product_id, user_id, rating, comment) VALUES('$product_id', '$user_id', '$rating', '$comment')";
mysqli_query($con, $sql);
?>