class UserRepository
{
    private $users = [];

    function __construct(UserDataMapper $dm){
        $this->dataMapper = $dm;
    }

    function getByID($id){
        if(!isset($this->users[$id])){
            $this->users[$id] = $this->dataMapper->getByID($id);
        }
        return $this->users[$id];
    }
}


$dm = new MysqlUserDataMapper($..., $...);

$repo = new UserRepository($dm);

$user = $repo->getByID(232);