How can the principles of object-oriented programming be effectively applied to create a login system with role-based access control in PHP?

```php
<?php

class User {
    private $username;
    private $password;
    private $role;

    public function __construct($username, $password, $role) {
        $this->username = $username;
        $this->password = $password;
        $this->role = $role;
    }

    public function getUsername() {
        return $this->username;
    }

    public function getPassword() {
        return $this->password;
    }

    public function getRole() {
        return $this->role;
    }
}

class Role {
    private $name;
    private $permissions;

    public function __construct($name, $permissions) {
        $this->name = $name;
        $this->permissions = $permissions;
    }

    public function getName() {
        return $this->name;
    }

    public function getPermissions() {
        return $this->permissions;
    }
}

class LoginSystem {
    private $users = [];

    public function addUser(User $user) {
        $this->users[$user->getUsername()] = $user;
    }

    public function login($username, $password) {
        if (isset($this->users[$username]) && $this->users[$username]->getPassword() == $password) {
            return $this->users[$username];
        }
        return null;
    }

    public function checkPermission(User $user, $permission) {
        $role = $user->getRole();
        if ($role && isset($this->roles[$role]) && in_array($permission, $this->roles[$role]->getPermissions())) {
            return true;
        }
        return false;
    }
}

// Example usage
$adminRole = new Role('admin', ['manage_users', 'manage_roles']);
$userRole = new Role('user', ['view_content']);

$user1 = new User('admin', 'admin123', 'admin');
$user2 = new User('user', 'user123', 'user');

$loginSystem = new LoginSystem();
$loginSystem->addUser($user1);
$loginSystem->add