理解 PHP 中的 SOLID 原则

单一职责原则（SRP）

一个类应该只有一个改变的原因。

// Bad
class User {
    public function saveUser($data) { /* ... */ }
    public function generateReport() { /* ... */ }
    public function sendEmail() { /* ... */ }
}

// Good
class User {
    public function save($data) { /* ... */ }
}

class UserReportGenerator {
    public function generate() { /* ... */ }
}

class UserNotifier {
    public function sendEmail() { /* ... */ }
}

开放/封闭原则（OCP）

软件实体应该对扩展开放，对修改关闭。

// Bad
class PaymentProcessor {
    public function processPayment($type) {
        if ($type === 'credit') {
            // Process credit payment
        } elseif ($type === 'debit') {
            // Process debit payment
        }
    }
}

// Good
interface PaymentMethod {
    public function processPayment();
}

class CreditPayment implements PaymentMethod {
    public function processPayment() { /* ... */ }
}

class DebitPayment implements PaymentMethod {
    public function processPayment() { /* ... */ }
}

里氏替换原则 (LSP)

子类型必须可以替代其基类型。

// Bad
class Bird {
    public function fly() { /* ... */ }
}

class Penguin extends Bird {
    public function fly() {
        throw new Exception("Cannot fly");
    }
}

// Good
interface Flyable {
    public function fly();
}

class Bird {
    public function eat() { /* ... */ }
}

class Sparrow extends Bird implements Flyable {
    public function fly() { /* ... */ }
}

class Penguin extends Bird {
    public function swim() { /* ... */ }
}

接口隔离原则 (ISP)

客户端不应被迫依赖他们不使用的接口。

// Bad
interface Worker {
    public function work();
    public function eat();
    public function sleep();
}

// Good
interface Workable {
    public function work();
}

interface Eatable {
    public function eat();
}

interface Sleepable {
    public function sleep();
}

class Human implements Workable, Eatable, Sleepable {
    public function work() { /* ... */ }
    public function eat() { /* ... */ }
    public function sleep() { /* ... */ }
}

class Robot implements Workable {
    public function work() { /* ... */ }
}

依赖倒置原则 (DIP)

高级模块不应该依赖于低级模块。两者都应该依赖于抽象。

// Bad
class UserService {
    private MySQLDatabase $database;

    public function __construct() {
        $this->database = new MySQLDatabase();
    }
}

// Good
interface DatabaseInterface {
    public function query($sql);
}

class UserService {
    private DatabaseInterface $database;

    public function __construct(DatabaseInterface $database) {
        $this->database = $database;
    }
}

class MySQLDatabase implements DatabaseInterface {
    public function query($sql) { /* ... */ }
}

class PostgreSQLDatabase implements DatabaseInterface {
    public function query($sql) { /* ... */ }
}

这些原则有助于创建可维护、灵活且强大的代码。遵循 SOLID 可让您的代码更易于测试、修改和扩展。