Observer Pattern: How to Decouple Models from Side Effects in PHP MVC

The Observer Pattern is a behavioral design principle that establishes a one-to-many dependency between objects, so that when one object changes state, all registered dependents are notified automatically. In practice, this means a model's save() method no longer needs to directly handle cache invalidation, audit logging, or notifications — each concern is delegated to a separate observer. A concrete example is the QueryCacheObserver, which automatically invalidates only the relevant cache keys when a model is saved or deleted, without the model or controller being aware of the cache layer. The pattern is distinct from Publish-Subscribe, which adds a message broker intermediary so publishers and subscribers have no direct knowledge of each other. By inverting dependencies and isolating responsibilities, the Observer Pattern allows new behaviors to be added simply by registering a new observer, with no changes required to the subject.