PHP json object

Dear Experts,
I use PHP,
I need to get a result inside an object which is inside Json.
I get the parrent result like below.

curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

$server_output = curl_exec($ch);

var_dump ( $obj = json_decode($server_output));

echo $yeniobj= ($obj->result);

//Result is 
object(stdClass)#1 (10) { ["result"]=> string(90) "{"ReturnCode":0,"ReturnData":null,"Confirm":null,"TimeStamp":"04.11.2019 10:43:34"}" ["id"]=> int(135) ["exception"]=> NULL ["status"]=> int(5) ["isCanceled"]=> bool(false) ["isCompleted"]=> bool(true) ["isCompletedSuccessfully"]=> bool(true) ["creationOptions"]=> int(0) ["asyncState"]=> NULL ["isFaulted"]=> bool(false) }

echo $yeniobj= ($obj->result);

//Result is

{"ReturnCode":0,"ReturnData":null,"PaymentConfirm":null,"TimeStamp":"04.11.2019 10:43:34"}

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But I need to result of the ReturnCode information.
How can I get that? thank you