如何利用PHP函数实现电子商务功能？

<?php
$servername = "localhost";
$username = "root";
$password = "password";
$dbname = "myDB";

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

// 检查连接是否成功
if ($conn->connect_error) {
    die("连接数据库失败：" . $conn->connect_error);
} else {
    echo "成功连接到数据库！";
}

// 关闭连接
$conn->close();
?>

<?php
// 用户注册
if(isset($_POST['register'])){
    $username = $_POST['username'];
    $password = $_POST['password'];
    
    // 将密码进行加密处理
    $hashed_password = password_hash($password, PASSWORD_DEFAULT);
    
    // 将用户名和加密后的密码保存到数据库
    $sql = "INSERT INTO users (username, password) VALUES ('$username', '$hashed_password')";
    
    // 执行SQL语句
    if ($conn->query($sql) === TRUE) {
        echo "用户注册成功！";
    } else {
        echo "用户注册失败：" . $conn->error;
    }
}

// 用户登录
if(isset($_POST['login'])){
    $username = $_POST['username'];
    $password = $_POST['password'];
    
    // 查询数据库获取用户信息
    $sql = "SELECT password FROM users WHERE username='$username'";
    $result = $conn->query($sql);
    
    if ($result->num_rows > 0) {
        $row = $result->fetch_assoc();
        
        // 验证密码是否正确
        if(password_verify($password, $row['password'])){
            echo "用户登录成功！";
        } else {
            echo "密码错误！";
        }
    } else {
        echo "用户不存在！";
    }
}

// 关闭数据库连接
$conn->close();
?>

<?php
// 获取商品列表
$sql = "SELECT * FROM products";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
        echo "商品名称：" . $row["name"]. " - 商品价格：" . $row["price"]. "<br>";
        
        // 添加到购物车
        echo "<form method='post' action=''>
        <input type='hidden' name='product_id' value='".$row["id"]."'>
        <input type='number' name='quantity' value='1'>
        <input type='submit' name='add_to_cart' value='加入购物车'>
        </form>";
    }
} else {
    echo "暂无商品！";
}

// 购物车管理
session_start();
if(isset($_POST['add_to_cart'])){
    $product_id = $_POST['product_id'];
    $quantity = $_POST['quantity'];
    
    $_SESSION['cart'][$product_id] = $quantity;
    
    echo "商品已添加到购物车！";
}

// 关闭数据库连接
$conn->close();
?>