你用PHP写一套云笔记后端源代码

好的，以下是一个基于PHP的云笔记后端的示例代码，供您参考：

<?php

// 连接数据库
$host = "localhost";
$username = "your_username";
$password = "your_password";
$database = "your_database";
$conn = mysqli_connect($host, $username, $password, $database);

// 用户注册
function register($username, $password) {
    global $conn;
    $password = md5($password); // 使用md5加密密码
    $sql = "INSERT INTO users (username, password) VALUES ('$username', '$password')";
    mysqli_query($conn, $sql);
}

// 用户登录
function login($username, $password) {
    global $conn;
    $password = md5($password); // 使用md5加密密码
    $sql = "SELECT * FROM users WHERE username='$username' AND password='$password'";
    $result = mysqli_query($conn, $sql);
    $user = mysqli_fetch_assoc($result);
    return $user;
}

// 新建笔记
function create_note($title, $content, $category_id, $user_id) {
    global $conn;
    $sql = "INSERT INTO notes (title, content, category_id, user_id) VALUES ('$title', '$content', '$category_id', '$user_id')";
    mysqli_query($conn, $sql);
}

// 获取笔记列表
function get_note_list($user_id) {
    global $conn;
    $sql = "SELECT * FROM notes WHERE user_id='$user_id'";
    $result = mysqli_query($conn, $sql);
    $notes = mysqli_fetch_all($result, MYSQLI_ASSOC);
    return $notes;
}

// 获取笔记详情
function get_note_detail($note_id) {
    global $conn;
    $sql = "SELECT * FROM notes WHERE id='$note_id'";
    $result = mysqli_query($conn, $sql);
    $note = mysqli_fetch_assoc($result);
    return $note;
}

// 删除笔记
function delete_note($note_id) {
    global $conn;
    $sql = "DELETE FROM notes WHERE id='$note_id'";
    mysqli_query($conn, $sql);
}

// 定义API接口
if ($_SERVER['REQUEST_METHOD'] === 'POST' && $_GET['action'] === 'register') {
    $username = $_POST['username'];
    $password = $_POST['password'];
    register($username, $password);
} elseif ($_SERVER['REQUEST_METHOD'] === 'POST' && $_GET['action'] === 'login') {
    $username = $_POST['username'];
    $password = $_POST['password'];
    $user = login($username, $password);
    echo json_encode($user);
} elseif ($_SERVER['REQUEST_METHOD'] === 'POST' && $_GET['action'] === 'create_note') {
    $title = $_POST['title'];
    $content = $_POST['content'];
    $category_id = $_POST['category_id'];
    $user_id = $_POST['user_id'];
    create_note($title, $content, $category_id, $user_id);
} elseif ($_SERVER['REQUEST_METHOD'] === 'GET' && $_GET['action'] === 'get_note_list') {
    $user_id = $_GET['user_id'];
    $notes = get_note_list($user_id);
    echo json_encode($notes);
} elseif ($_SERVER['REQUEST_METHOD'] === 'GET' && $_GET['action'] === 'get_note_detail') {
    $note_id = $_GET['note_id'];
    $note = get_note_detail($note_id);
    echo json_encode($note);
} elseif ($_SERVER['REQUEST_METHOD'] === 'POST' && $_GET['action'] === 'delete_note') {
    $note_id = $_POST['note_id'];
    delete_note($note_id);
}

?>

这是一个简单的示例代码，仅供参考。实际项目中需要考虑更多的安全性和性能优化问题，比如SQL注入、XSS攻击等。如果您需要更完整的代码，可以在网上查找相关的开源项目或者使用框架来实现。