Как отправить JSON с android to PHP?

php

$firstname=$_POST['firstname'];
print (json_decode($firstname));

android

try{
    URL url1=new URL(url2); 
    HttpURLConnection conn1=(HttpURLConnection)url1.openConnection();
    conn1.setReadTimeout(10000);
    conn1.setConnectTimeout(15000);
    conn1.setDoOutput(true);
    conn1.setRequestMethod("POST");
    conn1.connect();
    OutputStream outputStream=conn1.getOutputStream();
    BufferedWriter writer=new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8") );
    JSONObject json1=new JSONObject();
    json1.put("firstname", "name");
    writer.write(URLEncoder.encode(json1.toString()));
    writer.close();
    outputStream.close();
    InputStream in=conn1.getInputStream();
    String data=convertStreamToString(in);
    Log.d("in", data);
}catch(Exception e){
    Log.d("logdb", "error");
}

выводит ошибку Undefined index: firstname