public function userDetails(){

    return $this->hasOne(UserDetail::class);

}

public function index()
{
     $user  = User::find(2);
     $user->user_details = $user->userDetails;
     echo "<pre>"; print_r($user->toArray()); die;
}

Array
(
    [id] => 2
    [name] => Joney
    [email] => joney@gmail.com
    [email_verified_at] => 2019-04-29 02:01:03
    [password] => dasDSADASDASDAS
    [status] => Active
    [remember_token] => DSADASd
    [created_at] => 2019-04-29 00:00:00
    [updated_at] => 2018-09-24 02:00:00
    [user_details] => Array
        (
            [id] => 2
            [user_id] => 2
            [address] => Dhampur
            [mobile_number] => 8006009195
            [passport_number] => sasdadasd
            [zip_code] => 201301
            [created_at] => 2019-04-29 06:00:00
            [updated_at] => 2019-04-29 02:00:00
        )

)

$user  = User::with('userDetails')->find(2);
    $user->user_detail_id = $user->userDetails->id;
    $user->address = $user->userDetails->address;
    $user->mobile_number = $user->userDetails->mobile_number;
    $user->passport_number = $user->userDetails->passport_number;
    $user->zip_code = $user->userDetails->zip_code;
    unset($user->userDetails);
    echo "<pre>"; print_r($user->toArray()); die;