Applying the Observer Pattern with PHP Events

An observer receives events from a publisher without coupling the publisher to every downstream action.

What It Does and When to Use It

Implement a simple dispatcher with separately registered listeners. Direct PHP code exposes the underlying operation clearly; a framework can supply lifecycle hooks, dependency wiring and retries.

Requirements, Inputs and Expected Behavior

Confirm the runtime, extension, environment variables, and permissions before running the examples in production. Keep listeners predictable, isolate failures and avoid recursive event chains.

Example 1: Basic Operation

Implement a simple dispatcher with separately registered listeners.

<?php
final class Dispatcher {
     private array $listeners = [];
     public function on(string $name, callable $listener): void { $this->listeners[$name][] = $listener; }
     public function dispatch(string $name, object $event): void {
         foreach ($this->listeners[$name] ?? [] as $listener) $listener($event);
     }
 }
 $events = new Dispatcher();
 $events->on('invoice.created', static function (object $e): void { echo $e->id; });
 $events->dispatch('invoice.created', (object)['id' => 23]);

Example 2: Behavior and Validation

Separate several reactions from the action that published an event.

<?php
$handlers = [static fn (int $id) => "audit:$id", static fn (int $id) => "email:$id"];
 $results = array_map(static fn (callable $f) => $f(42), $handlers);
 print_r($results);

Practical Example: Application Use

Pass structured event data and keep validation explicit.

<?php
$listeners = [];
 $listeners['order.paid'][] = static function (array $event): void {
     if (($event['amount'] ?? 0) <= 0) throw new InvalidArgumentException('Positive amount required');
     echo 'Recorded payment ', $event['id'];
 };
 foreach ($listeners['order.paid'] as $listen) $listen(['id' => 'p-13', 'amount' => 85]);

Important Behavior and Edge Cases

The second example highlights: Separate several reactions from the action that published an event. The practical example adds: Pass structured event data and keep validation explicit.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• An observer receives events from a publisher without coupling the publisher to every downstream action.
• Pass structured event data and keep validation explicit.
• Choose based on deployment requirements and verify actual behavior rather than assuming defaults.