我如何json编码PHP中的私有属性？

class User {
    private $name;
    private $email;

    public function __construct($name, $email) {
        $this->name = $name;
        $this->email = $email;
    }

    public function getName() {
        return $this->name;
    }

    public function getEmail() {
        return $this->email;
    }
}

$user = new User("John Doe", "john.doe@example.com");
$name = $user->getName();
$email = $user->getEmail();

$json = json_encode(array(
    "name" => $name,
    "email" => $email
));

echo $json;

{
    "name": "John Doe",
    "email": "john.doe@example.com"
}