PHP Observer Pattern

What It Is

The Observer pattern lets one object announce events to many subscribers without knowing who they are. Subscribers register once; from then on, every event reaches all of them automatically.

The publisher depends only on a small subscriber interface — new kinds of listeners appear without one line changing in the publisher. PHP even ships ready-made SplSubject and SplObserver interfaces for this pattern; a small interface of your own, as below, reads just as well.

Example: One Alert, Every Subscriber

The monitor keeps a list of subscribers. When something happens, it notifies the whole list — an email alert and a dashboard, today; a chat message tomorrow, by just subscribing it.

Publish once, deliver to all

One alert() call, two deliveries. The monitor has no idea what an email or a dashboard is — it only knows things that can be notified.

Write a PHP script with the Observer pattern: a Subscriber interface with notify, an EmailAlert and a Dashboard subscriber, and a Monitor publisher with subscribe and alert methods that notifies every subscriber of an event. Subscribe both and raise one alert.

> Output: PHP script

Prompt

Write a PHP script with the Observer pattern: a Subscriber interface with notify, an EmailAlert and a Dashboard subscriber, and a Monitor publisher with subscribe and alert methods that notifies every subscriber of an event. Subscribe both and raise one alert.

Output: PHP script

<?php

interface Subscriber {
  public function notify(string $event): void;
}

class EmailAlert implements Subscriber {
  public function __construct(private string $addr) {}

  public function notify(string $event): void {
    echo "email to ", $this->addr, ": ", $event, "\n";
  }
}

class Dashboard implements Subscriber {
  public function notify(string $event): void {
    echo "dashboard shows: ", $event, "\n";
  }
}

// The publisher keeps a list and notifies everyone on it.
class Monitor {
  /** @var Subscriber[] */
  private array $subscribers = [];

  public function subscribe(Subscriber $s): void {
    $this->subscribers[] = $s;
  }

  public function alert(string $event): void {
    foreach ($this->subscribers as $s) {
      $s->notify($event);
    }
  }
}

$monitor = new Monitor();
$monitor->subscribe(new EmailAlert("ops@example.com"));
$monitor->subscribe(new Dashboard());

$monitor->alert("disk almost full");

When to Use It

Use it when several parts must react to the same events and the list of reactors changes — UI updates, alerting, cache invalidation, activity feeds.

Skip it when exactly one, fixed receiver exists — call it directly. An event dispatcher in a larger PHP application is this same pattern grown up: named events, many listeners, one dispatch point.

Pros and Cons

Pros: publisher and subscribers stay decoupled; subscribers come and go at run time; new reactions need no publisher changes.

Cons: the flow becomes invisible — "who reacts to this?" takes searching; a slow or failing subscriber can stall the notify loop; forgotten unsubscribes leak.