class Event {
        protected $_data_mapper;

        public function __construct( EventDataMapper $data_mapper ) {
            $this->_data_mapper = $data_mapper;
        }
    }

    class EventDataMapper {
        protected $_db;

        public function __construct( Database $db ) {
            $this->_db = $db;
            return $this;
        }

        public function find( int $id ) {
            // do the query
            $event = new Event( $this );
            // set all the properties
            return $event;
        }
    }

    $loaded_event = ( new EventDataMapper( $db ) )->find( $event_id );