DonationEvents.php

<?php

class DonationEvents {

    private $donationCreatedCallbacks = [];
    private $lolCatAddedCallbacks = [];

    public function onDonationCreated( callable $callback ) {
        $this->donationCreatedCallbacks[] = $callback;
    }

    public function onLolCatAdded( callable $callback ) {
        $this->lolCatAddedCallbacks[] = $callback;
    }

    public function donationCreated( DonationCreatedEvent $createdEvent ) {
        foreach ( $this->donationCreatedCallbacks as $callback ) {
            $callback( $createdEvent );
        }
    }

    public function lolCatAdded() {
        foreach ( $this->lolCatAddedCallbacks as $callback ) {
            $callback();
        }
    }

}

class DonationCreatedEvent {

    public function getDonationId(): int {
    }

}

class EventRegisterer {

    /**
     * Called during application initialization
     */
    public function register( DonationEvents $events ) {
        $events->onDonationCreated( function() {
            $this->factory->getFirewroksService()->launchFireworks();
        } );

        $events->onDonationCreated( function( DonationCreatedEvent $event ) {
            $this->factory->getDonationCreatedMailer()->mailConfirmation( $event->getDonationId() );
        } );
        
        // Might make sense to just have onDonationCreated here. 2 for demonstration purpouses
    }

}

class SuchUseCase {

    private $events;

    public function __construct( DonationEvents $events ) {
        $this->events = $events;
    }

    public function doSuch() {
        $donation = $this->createDonation();

        $this->events->donationCreated( new DonationCreatedEvent( $donation->getId() ) );
    }

}