Отправка данных POST в Android для переполнения стека

Я ищу способ отправки данных POST в скрипт PHP и отображения результата. Я хочу отправить данные POST $ username на php.
Это мой php файл:

<?php

...// connect

$username = $_POST['txtUsername'];// this data

$mysqli->query("SET NAMES 'utf8'");
$sql="SELECT * FROM $username";
$result=$mysqli->query($sql);
while($e=mysqli_fetch_assoc($result)){
$output[]=$e;
}

print(json_encode($output));
$mysqli->close();

?>

Моя деятельность:

DataParser.class

public class DataParser extends AsyncTask<Void,Void,Integer>{

Context c;
ListView lv;
String jsonData;

ProgressDialog pd;
ArrayList<Spacecraft> spacecrafts = new ArrayList<>();

public DataParser(Context c, ListView lv, String jsonData) {
this.c = c;
this.lv = lv;
this.jsonData = jsonData;
}

@Override
protected void onPreExecute() {
super.onPreExecute();

pd= new ProgressDialog(c);
pd.setTitle("Parse");
pd.setMessage("Parsing..Please wait");
pd.show();
}

@Override
protected Integer doInBackground(Void... voids) {
return this.parseData();
}

@Override
protected void onPostExecute(Integer result) {
super.onPostExecute(result);pd.dismiss();
if (result==0){
Toast.makeText(c,"Unable to parse",Toast.LENGTH_SHORT).show();
}
else {
CustomAdapter adapter = new CustomAdapter(c,spacecrafts);
lv.setAdapter(adapter);
}
}
private  int parseData(){
try {
JSONArray ja = new JSONArray(jsonData);
JSONObject jo = null;

spacecrafts.clear();
Spacecraft s=null;for (int i=0;i<ja.length();i++){
jo=ja.getJSONObject(i);
int id = jo.getInt("id");
String name =jo.getString("name");
String send =jo.getString("send");
String comment =jo.getString("comment");
String date =jo.getString("date");

s=new Spacecraft();
s.setId(id);
s.setName(name);
s.setSend(send);
s.setComment(comment);
s.setDate(date);

spacecrafts.add(s);
}return 1;
} catch (JSONException e) {
e.printStackTrace();
}

return 0;
}}

Основная деятельность:

public class MainActivity extends AppCompatActivity implements AsyncResponse {

String urlAddress="http://mysite/myphp.php";

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

final ListView lv = (ListView) findViewById(R.id.lv) ;

Intent intent = getIntent();
final String username = intent.getStringExtra("location");

Downloader d = new Downloader(MainActivity.this,urlAddress,lv);
d.execute();

И Downloader.class:

public class Downloader extends AsyncTask<Void,Void,String> {

Context c;
String urlAddress;
ListView lv;

ProgressDialog pd;
public Downloader(Context c, String urlAddress, ListView lv) {
this.c = c;
this.urlAddress = urlAddress;
this.lv = lv;
}

@Override
protected void onPreExecute() {
super.onPreExecute();

pd = new ProgressDialog(c);
pd.setTitle("Fetch");
pd.setMessage("Fetching...Please wait");
pd.show();
}

@Override
protected String doInBackground(Void... voids) {
return this.downloadData();
}

@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);

pd.dismiss();

if(s==null){
Toast.makeText(c,"Unsuccessful,Null,returned",Toast.LENGTH_SHORT).show();
}
else {
DataParser parser = new DataParser(c,lv,s);
parser.execute();

}

}private String downloadData(){
HttpURLConnection con = Connector.connect(urlAddress);
if (con==null){
return null;
}
InputStream is=null;
try {

is=new BufferedInputStream(con.getInputStream());
BufferedReader br = new BufferedReader(new InputStreamReader(is));

String line = null;
StringBuffer response = new StringBuffer();

if (br!=null){
while ((line=br.readLine())!=null){
response.append(line+"\n");
}

br.close();
}
else {return null;}
return response.toString();

} catch (IOException e) {
e.printStackTrace();
}
finally {
if(is != null){
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}

return null;
}}

Пожалуйста, помогите мне, спасибо!