events.php

<?php
$dispatcher = new EventDispatcher;


class SendWelcomeEmail implements QueuedEventHandler {
    
    public function when(Event $event) {
        // delegate to whenUserHasRegistered, whenFoo..
    }
    private function whenUserHasRegistered(UserHasRegistered $event) {
       // do stuff with $event->getUserId();
    }
}

$dispatcher->listen(UserHasRegistered::class, $sendWelcomeEmail);

$dispatcher->fire(new UserHasRegistered($userid, ...));