How can PHP developers ensure that their modular code remains maintainable and scalable over time?

To ensure that modular PHP code remains maintainable and scalable over time, developers should follow best practices such as using namespaces, separating concerns into different files or classes, utilizing autoloading, and implementing design patterns like MVC. Additionally, documenting code, writing unit tests, and adhering to coding standards can help improve code readability and maintainability.

// Example of using namespaces and separating concerns into different files

// File: User.php
namespace MyApp\Models;

class User {
    // Class implementation
}

// File: UserController.php
namespace MyApp\Controllers;

use MyApp\Models\User;

class UserController {
    // Class implementation
}