JsonConvert.SerializeObject vs json_encode

I am Converting asp.net c# application into php i am facing problem with json encode like i have to encode one datetime field like created date into php so i am using simply

$arr=array('OrderDate'=>date('Y-m-d H:i:s'));
echo `json_encode($arr);`
//output => {"OrderDate": "2017-03-22 06:21:28"}

same thing in asp.net

Order objOrder= new Order();
objOrder.OrderDate = SystemTime.Now();
JsonConvert.SerializeObject($objOrder); 

returns

{"OrderDate": "2017-03-22T10:40:17.313313+05:30"}

i wanted to exactly same output like asp.net using php json_encode() how do i achieve?

$arr=array('OrderDate'=>date('Y-m-d\TH:i:s.uP'));
echo json_encode($arr);

How about this?

$arr=array('OrderDate'=>date('Y-m-dTH:i:s.uP'));
echo `json_encode($arr);`

or

$arr=array('OrderDate'=>date('c'));
echo `json_encode($arr);`

I found php date format.