# PHP Observer Pattern

The **Observer** pattern is a behavioral design pattern that lets you define a subscription mechanism to notify multiple objects about any events that happen to the object they're observing.

---

## 1. Concept
*   **Subject (Publisher):** The object that has interesting state. It maintains a list of observers and notifies them when state changes.
*   **Observer (Subscriber):** The objects that want to be notified when the Subject's state changes.

## 2. Implementation using SPL
PHP provides built-in interfaces `SplSubject` and `SplObserver` to implement this pattern easily.

### The Subject
```php
class User implements SplSubject {
    public $email;
    private $observers;

    public function __construct($email) {
        $this->email = $email;
        $this->observers = new SplObjectStorage();
    }

    public function attach(SplObserver $observer) {
        $this->observers->attach($observer);
    }

    public function detach(SplObserver $observer) {
        $this->observers->detach($observer);
    }

    public function notify() {
        foreach ($this->observers as $observer) {
            $observer->update($this);
        }
    }

    public function changeEmail($newEmail) {
        $this->email = $newEmail;
        $this->notify();
    }
}
```

### The Observer
```php
class LogHandler implements SplObserver {
    public function update(SplSubject $subject) {
        echo "LogHandler: User changed email to " . $subject->email . "\n";
    }
}
```

### Usage
```php
$user = new User("john@example.com");
$logger = new LogHandler();

$user->attach($logger);

$user->changeEmail("john.doe@gmail.com");
// Outputs: LogHandler: User changed email to john.doe@gmail.com
```

[[programming/php/php]] [[programming/php/php-design-patterns]]