Need to display only array value in JSON output
echo '{"aaData":'.json_encode($user_details).'}';

{"aaData": [
    {"id":"31","name":"Elankeeran","email":"ekeeran@yahoo.com","activated":"0","phone":""}
]}

{"aaData": [
    {"31","Elankeeran","ekeeran@yahoo.com","0","1234"}
]}

$rows = array();
foreach ($user_details as $row) {
  $rows[] = array_values((array)$row);
}

echo json_encode(array('aaData'=> $rows));

{"aaData": [
    ["31","Elankeeran","test@yahoo.com","0","1234"],
    ["33","Elan","test@gmail.com","1",""]
]}

echo '{"aaData":'.json_encode(array_values($user_details)).'}';

obj.aaData[0].name;
// Elankeeran

obj.aaData[0].email;
// email@yahoo.com