问为什么PHP私有变量在扩展类中被公开

EN

abstract class Model
{
    var $id;

    private $cName;
    private $tName;

    public function __construct($id = 0)
    {
        $this->cName = 'Im cName';
        $this->tName = 'Im tName';            
    }
}

class claseExtend extends Model
{
    var $id;

    public function hola()
    {
        $this->id = 1;
        return (array) $this;
    }
}

$obj = new claseExtend() ;
$retHola =$obj->hola();
print_r($retHola);