### Instruction:
Develop a PHP web application that allows users to create, read, update, and delete notes stored in a MySQL database, with user authentication and access control implemented.


### Response:

To create a PHP web application that allows users to create, read, update, and delete (CRUD) notes stored in a MySQL database with user authentication and access control implemented, follow these steps:

1. Set up the development environment:
   - Install XAMPP or any other local server that supports PHP and MySQL.
   - Create a new folder for your project in the "htdocs" directory of your local server.

2. Create a MySQL database and tables:
   - Open phpMyAdmin from your local server dashboard and create a new database named "notes_app".
   - In this database, create two tables: "users" and "notes".

3. Design the table structure as follows:

   Users table:
   - id (INT, PRIMARY KEY, AUTO_INCREMENT)
   - username (VARCHAR)
   - email (VARCHAR)
   - password (VARCHAR)

   Notes table:
   - id (INT, PRIMARY KEY, AUTO_INCREMENT)
   - user_id (INT, FOREIGN KEY REFERENCES users(id))
   - title (VARCHAR)
   - content (TEXT)
   - created_at (DATETIME)
   - updated_at (DATETIME)

4. Create a configuration file (config.php) to store database connection details:

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

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}
?>
```

5. Create a registration form (register.php) for users to sign up:

```html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Register</title>
</head>
<body>
    <h1>Register</h1>
    <form action="register_action.php" method="post">
        <label for="username">Username:</label>
        <input type="text" name="username" id="username" required><br>

        <label for="email">Email:</label>
        <input type="email" name="email" id="email" required><br>

        <label for="password">Password:</label>
        <input type="password" name="password" id="password" required><br>

        <button type="submit">Register</button>
    </form>
</body>
</html>
```

6. Create a PHP script (register_action.php) to handle user registration:

```php
<?php
require_once 'config.php';

$username = $_POST['username'];
$email = $_POST['email'];
$password = password_hash($_POST['password'], PASSWORD_DEFAULT);

$sql = "INSERT INTO users (username, email, password) VALUES (?, ?, ?)";
$stmt = $conn->prepare($sql);
$stmt->bind_param("sss", $username, $email, $password);

if ($stmt->execute()) {
    header("Location: login.php");
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$stmt->close();
$conn->close();
?>
```

7. Create a login form (login.php) for users to sign in:

```html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Login</title>
</head>
<body>
    <h1>Login</h1>
    <form action="login_action.php" method="post">
        <label for="email">Email:</label>
        <input type="email" name="email" id="email" required><br>

        <label for="password">Password:</label>
        <input type="password" name="password" id="password" required><br>

        <button type="submit">Login</button>
    </form>
</body>
</html>
```

8. Create a PHP script (login_action.php) to handle user authentication:

```php
<?php
session_start();
require_once 'config.php';

$email = $_POST['email'];
$password = $_POST['password'];

$sql = "SELECT * FROM users WHERE email = ?";
$stmt = $conn->prepare($sql);
$stmt->bind_param("s", $email);
$stmt->execute();
$result = $stmt->get_result();

if ($result->num_rows > 0) {
    $user = $result->fetch_assoc();
    if (password_verify($password, $user['password'])) {
        $_SESSION['user_id'] = $user['id'];
        header("Location: notes.php");
    } else {
        echo "Invalid password";
    }
} else {
    echo "User not found";
}

$stmt->close();
$conn->close();
?>
```

9. Create the main page (notes.php) to display and manage notes:

```html
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Notes</title>
</head>
<body>
    <h1>Your Notes</h1>
    <!-- Add your code here to display and manage notes -->
</body>
</html>
```

10. Implement CRUD operations for notes in the notes.php file.

11. To implement access control, add the following code at the beginning of each PHP file that requires user authentication:

```php
<?php
session_start();
if (!isset($_SESSION['user_id'])) {
    header("Location: login.php");
}
?>
```

This will redirect users to the login page if they are not logged in.

With these steps, you should have a basic PHP web application with CRUD functionality for notes and user authentication. You can further enhance the application by adding features like pagination, search, and sorting.