while ($row = $result->fetch_assoc()) {
    echo (json_encode($row));
}

func parseJSON(_ data:Data) {
    var jsonResult = NSArray()
    do {
        jsonResult = try JSONSerialization.jsonObject(with: data, options: []) as! NSArray
    } catch let error as NSError {
        print(error)
    }
}

{"userid":"8","name":"Sam","highscore":"215","rank":"1"}
{"userid":"9","name":"James","highscore":"210","rank":"2"}
{"userid":"10","name":"Julian","highscore":"162","rank":"3"}