I have always kept an eye out for event systems when writing code in scripting languages. Web applications have a history of allowing the user to add plugins and modules whenever needed.

In most PHP systems you have a global/singleton event object which all interested parties tie into and wait to be alerted to changes.

Event::on('event_name', $callback);

Recently more patterns like the observer have been used for things like jQuery.

$(el).on('event', callback);

Even PHP now has built in classes for it.

class Blog extends SplSubject {
    public function save()
    {
        $this->notify();
    }
}

Anyway, the Promises/A proposal has caught my eye. It is designed for asynchronous systems, but I'm wondering if it is also a good design to implement now that even synchronous languages like PHP are changing.

Combining Dependency Injection with Promises/A seems it might be the best combination for handling events currently.