php+mysql实现一个日记管理系统，在这个系统中可以进行日记的添加、修改、删除、浏览、查询等操作，并实现密码保护和密码修改功能。并对日记信息按照日期进行排序，并将日记存储到文件中。代码

好的，下面是一个简单的 PHP + MySQL 日记管理系统的示例代码：

1. 首先是数据库连接代码：

<?php 
$servername = "localhost";  // 数据库服务器名
$username = "root";         // 数据库用户名
$password = "";             // 数据库密码
$dbname = "diary";          // 数据库名

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

// 检测连接是否成功
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}
?>

2. 创建一个表来存储日记信息：

CREATE TABLE diary (
  id INT(6) UNSIGNED AUTO_INCREMENT PRIMARY KEY,
  title VARCHAR(30) NOT NULL,
  content TEXT NOT NULL,
  password VARCHAR(50),
  create_time TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
)

3. 实现密码加密和解密：

<?php
// 密码加密函数
function encrypt_password($password) {
    $salt = "MyDiary";      // 加盐
    return md5($password.$salt);
}

// 密码解密函数（因为这里只是简单地对密码进行加密，所以不需要解密函数）
?>

4. 实现日记的添加、修改、删除、浏览和查询：

<?php
// 添加日记
function add_diary($title, $content, $password) {
    global $conn;
    $title = mysqli_real_escape_string($conn, $title);       // 防止 SQL 注入
    $content = mysqli_real_escape_string($conn, $content);
    $password = encrypt_password($password);                // 加密密码

    $sql = "INSERT INTO diary (title, content, password) VALUES ('$title', '$content', '$password')";

    if (mysqli_query($conn, $sql)) {
        return true;
    } else {
        return false;
    }
}

// 修改日记
function update_diary($id, $title, $content, $password) {
    global $conn;
    $title = mysqli_real_escape_string($conn, $title);
    $content = mysqli_real_escape_string($conn, $content);
    $password = encrypt_password($password);

    $sql = "UPDATE diary SET title='$title', content='$content', password='$password' WHERE id=$id";

    if (mysqli_query($conn, $sql)) {
        return true;
    } else {
        return false;
    }
}

// 删除日记
function delete_diary($id) {
    global $conn;

    $sql = "DELETE FROM diary WHERE id=$id";

    if (mysqli_query($conn, $sql)) {
        return true;
    } else {
        return false;
    }
}

// 获取所有日记
function get_all_diaries() {
    global $conn;

    $sql = "SELECT * FROM diary ORDER BY create_time DESC";

    $result = mysqli_query($conn, $sql);

    if (mysqli_num_rows($result) > 0) {
        $diaries = array();
        while($row = mysqli_fetch_assoc($result)) {
            $diaries[] = $row;
        }
        return $diaries;
    } else {
        return null;
    }
}

// 获取一篇日记
function get_diary($id, $password) {
    global $conn;
    $password = encrypt_password($password);

    $sql = "SELECT * FROM diary WHERE id=$id AND password='$password'";

    $result = mysqli_query($conn, $sql);

    if (mysqli_num_rows($result) == 1) {
        $diary = mysqli_fetch_assoc($result);
        return $diary;
    } else {
        return null;
    }
}

// 搜索日记
function search_diaries($keyword) {
    global $conn;
    $keyword = mysqli_real_escape_string($conn, $keyword);

    $sql = "SELECT * FROM diary WHERE title LIKE '%$keyword%' ORDER BY create_time DESC";

    $result = mysqli_query($conn, $sql);

    if (mysqli_num_rows($result) > 0) {
        $diaries = array();
        while($row = mysqli_fetch_assoc($result)) {
            $diaries[] = $row;
        }
        return $diaries;
    } else {
        return null;
    }
}
?>

5. 实现密码保护和密码修改功能：

<?php
// 检查密码是否正确
function check_password($id, $password) {
    global $conn;
    $password = encrypt_password($password);

    $sql = "SELECT * FROM diary WHERE id=$id AND password='$password'";

    $result = mysqli_query($conn, $sql);

    if (mysqli_num_rows($result) == 1) {
        return true;
    } else {
        return false;
    }
}

// 修改密码
function change_password($id, $old_password, $new_password) {
    global $conn;
    $old_password = encrypt_password($old_password);
    $new_password = encrypt_password($new_password);

    if (check_password($id, $old_password)) {
        $sql = "UPDATE diary SET password='$new_password' WHERE id=$id";

        if (mysqli_query($conn, $sql)) {
            return true;
        } else {
            return false;
        }
    } else {
        return false;
    }
}
?>

6. 最后是一个简单的前端页面代码：

<!DOCTYPE html>
<html>
<head>
    <title>My Diary</title>
</head>
<body>
    <h1>My Diary</h1>
    <hr>
    <h2>Add a new diary</h2>
    <form action="add.php" method="post">
        Title: <input type="text" name="title"><br>
        Content: <br>
        <textarea name="content" rows="10" cols="50"></textarea><br>
        Password: <input type="password" name="password"><br>
        <input type="submit" value="Add">
    </form>
    <hr>
    <h2>All diaries</h2>
    <?php
    include 'functions.php';

    $diaries = get_all_diaries();

    if ($diaries == null) {
        echo "No diaries found.";
    } else {
        foreach ($diaries as $diary) {
            $id = $diary['id'];
            $title = $diary['title'];
            $content = $diary['content'];
            $create_time = $diary['create_time'];

            echo "<h3><a href='view.php?id=$id'>$title</a></h3>";
            echo "<p>$content</p>";
            echo "<p>Created time: $create_time</p>";
            echo "<hr>";
        }
    }
    ?>
</body>
</html>

这只是一个简单的示例代码，实际上还有很多细节需要注意，比如权限控制、防止 SQL 注入、前端验证等等。