Domain=NSCocoaErrorDomain Code=3840について

swift3.0でJSONを解析したいのですが下記のエラーが出てしまい困っています。
swift、PHPともに勉強中ですのでまだ理解してない部分も多いです。
このような説明で大変申し訳ないのですが、ご教示お願いいたします。

Error 43-> Error Domain=NSCocoaErrorDomain Code=3840 "JSON text did not start with array or object and option to allow fragments not set." UserInfo={NSDebugDescription=JSON text did not start with array or object and option to allow fragments not set.}

以下ソースコードになります。
.swift

let array = ["data=文字列"]

    do {
        let jsonData = try JSONSerialization.data(withJSONObject: array, options: .prettyPrinted)

        let request = NSMutableURLRequest(url: NSURL(string: "http://localhost/test.php")! as URL)
        request.httpMethod = "POST"
        request.setValue("application/json; charset=utf-8", forHTTPHeaderField: "Content-Type")
        request.httpBody = jsonData
        request.addValue("application/json", forHTTPHeaderField: "Content-Type")
        request.addValue("application/json", forHTTPHeaderField: "Accept")

        let task = URLSession.shared.dataTask(with: request as URLRequest){ data, response, error in
            if error != nil{
                print("Error 55 -> \(error)")
                return
            }
            do {
                let result = try JSONSerialization.jsonObject(with: data!)
                print("Result 34 -> \(result)")
            } catch {
                print("Error  43-> \(error)")
            }
        }
        task.resume()
    }
    catch {
        print(error)
    }

.php

$json = $_POST['data'];
header('Content-type: application/json');
echo json_encode($json);