为什么我的服务器php代码无法从我的iOS应用程序接收JSON对象？

I am trying to send a JSON object containing username and password info from an iOS app to my server for login. However, it seems like the php code never received the JSON object or decoded it correctly. I have tried many different ways to convert JSON object and send to server, but none of them succeeded. I am coding in swift 2 by the way. Any help? THANKS!

Here is my swift code:

let myUrl = NSURL(string: "myURL");
let request = NSMutableURLRequest(URL: myUrl!);
request.HTTPMethod = "POST";

let params : [String : AnyObject] = ["username": username!, "password": password!];
request.setValue("application/json; charset=utf-8", forHTTPHeaderField: "Content-Type");

do {
   let jsonData = try NSJSONSerialization.dataWithJSONObject(params, options: NSJSONWritingOptions());
    request.HTTPBody = jsonData;
    let jsonString = NSString(data: jsonData, encoding: NSUTF8StringEncoding)! as String
    print(jsonString);
    print(request.HTTPBody);

} catch {
    print(error)
}

let task = NSURLSession.sharedSession().dataTaskWithRequest(request, completionHandler: { (data: NSData?, response: NSURLResponse?, error: NSError?) -> Void in

    do{
        let json = try NSJSONSerialization.JSONObjectWithData(data!, options: NSJSONReadingOptions.MutableLeaves) as? NSDictionary

        print(response)
        if let parseJSON = json {
            let resultValue = parseJSON["type"] as? Int
            print("result: \(resultValue)")
        }
    } catch {
        print(error)
    }
})

task.resume()

And here is my php code:

require("Conn.php");
require("MySQLDao.php");

$fp = fopen("data.txt", "a+");

$data = file_get_contents('php://input');
$json = json_decode($data, false);
$username = $_POST['username'];
$password = $_POST['password'];

fwrite($fp, $data);
fwrite($fp, $json);
fwrite($fp, $username);
fwrite($fp, $password);
fclose($fp);
$returnValue = array();

if (empty($username) || empty($password)) {
    $returnValue["status"] = "error";
    $returnValue["message"] = "Missing required field";
    echo json_encode($returnValue);
    return;
}  

$dao = new MySQLDao();
$dao->openConnection();
$userType = $dao->passwordAuthentification($username, $password);

if ($userType != null) {
    $returnValue["type"] = (int)$userType;
    echo json_encode($returnValue);
} else {
    $returnValue["type"] = -1;
    $returnValue["message"] = "user is not found";
    echo json_encode($returnValue);
}

$dao->closeConnection();