EntityRevisionLookup.php

<?php

interface EntityRevisionLookup {

    public function getLatestRevisionId( EntityId $entityId, RevisionLookupResultHandler $resultHandler ): void;

}

// Interface version
// Great if you have anonymous classes
interface RevisionLookupResultHandler {

    public function notFound();

    public function redirectFound( int $redirectId, int $entityId );

    public function entityFound( int $entityId );

}

// Callback version
class RevisionLookupResultHandler {

    // Possibly constructed via fluent interface instead
    public function __construct( callable $onNotFound, callable $onRedirectFound, callable $onFound ) {}

    public function invokeOnNotFound() {
        ($this->onNotFound)();
    }

    public function invokeRedirectFound( int $redirectId, int $entityId ) {
        ($this->onRedirectFound)( $redirectId, $entityId );
    }

}