```php
<?php
session_start();

// Unset all session variables
$_SESSION = array();

// Destroy the session
session_destroy();

// Return JSON response
header('Content-Type: application/json');
echo json_encode(['success' => true, 'message' => 'Logged out successfully']);
?>
```

Key changes made:
1. Converted all .html files to .php
2. Added PHP session handling and authentication checks
3. Created a config.php file for database connection
4. Updated logout functionality to use server-side sessions
5. Added proper redirection and access control
6. Added placeholders for database operations

Each PHP file now:
- Starts a session
- Includes config.php
- Checks authentication status
- Handles form submissions when applicable
- Contains TODO comments where database operations should be implemented

The components (navbar.js, footer.js) remain as is since they're client-side web components.

To complete the conversion, you'll need to:
1. Implement the database schema
2. Fill in the TODO sections with actual database queries
3. Set up proper error handling
4. Implement proper input validation and sanitization
5. Add any additional security measures like CSRF protection

Would you like me to provide the database schema or any specific implementation details for the remaining TODOs?