<?php
// WHEEL CLASSES
class Wheel {
    public $id;
    public $radius;
    public $grip;
}
class WheelMapper {
    public $pdo;

    public function __constructor($pdo) {
        $this->pdo = $pdo;
    }

    public function fetch($wheelCollection) {
        $result = $this->pdo-> Query table `wheels` where `car_id` = $wheel->id
    }
}
class WheelCollection {

}

// CAR CLASSES
class Car {
    public $id;
    public $make;
    public $model;
    public $wheels;

    public function setId($car_id) {
        $this->id = $car_id;
    }

    public function addWheels($wheels) {
        $this->wheels = $wheels;
    }

    public function getWheelType() {

    }
}
class carMapper {
    public $pdo;

    public function __constructor($pdo) {
        $this->pdo = $pdo;
    }

    public function fetch($car) {
        $result = $this->pdo-> Query table `cars` where `car_id` = $car->id
        $car->make = $result['make'];
        $car->model = $result['model'];
    }
}

$car = new Car;
$car->setId(42);
$mapper = new CarMapper($pdo);
if ( $mapper->fetch($car) ) //if there was a car in DB
{
    $wheels = new WheelCollection;
    $otherMapper = new WheelMapper( $pdo );

    $car->addWheels( $wheels );

    $wheels->setType($car->getWheelType());
    // I am not a mechanic. There is probably some name for describing
    // wheels that a car can use
    $otherMapper->fetch( $wheels );
}

?>