json_encode returns false while return data from array,,

I am using php version 7.1.3 with the symfony version 4.2.12
here is the array data

$json = [
                'totalVehicle' => $totalVehicle,
                'totalPrice' => $totalPrice,
                'vehicles' => $xvehicles,
                'toDateTime' => $toDateTime,
                'fromDateTime' => $fromDateTime,
                'distanceMile' => $distanceMile,
                'toCity' => $toCity,
                'fromCity' => $trip->getFromPlace(),
                'trip' => $trip,
                'adaAssistance' => $request->get('ada', false),
                'vehiclesCount' => $vehiclesCount,
                'pageCurrent' => $request->get('page', 0),
                'uri' => $uri,
                'maxPage' => floor($vehiclesCount / 15),
                'isSubscribed' => $isSubscribed,
            ];

when i use dd($json) it returns the actual array,

dd($json);

enter image description here

when i want to return it in json format by using json_encode it return false,

$json = json_encode($json,0,512);

dd($json) after json_encode
enter image description here

I tried to change the depth upto 1024 but the result is same with false return,
Please help me out to solve this,