How can PHP developers incorporate OOP principles and design patterns like dependency injection to create more modular and flexible code structures in their projects?

PHP developers can incorporate OOP principles and design patterns like dependency injection by creating classes that have a single responsibility, are loosely coupled, and follow the SOLID principles. By using dependency injection, developers can inject dependencies into classes rather than hardcoding them, making the code more modular and flexible.

// Example of dependency injection in PHP

class Database {
    public function query($sql) {
        // Database query logic
    }
}

class UserRepository {
    private $db;

    public function __construct(Database $db) {
        $this->db = $db;
    }

    public function getUserById($id) {
        return $this->db->query("SELECT * FROM users WHERE id = $id");
    }
}

// Implementation
$db = new Database();
$userRepository = new UserRepository($db);
$user = $userRepository->getUserById(1);