Error org.json.JSONException

Witam. Tworzę aplikację na Androida. Ma ona za zadanie pobieranie danych z zewnętrznego serwera Bazy danych mysql.
Wszystko było dobrze, ale do czasu kompilacji. Wywaliło mi błąd:

Errororg.json.JSONException: Value<?xml of type java.lang.String cannot be converted to JSONObject

Łącze się z bazą danych za pomocą skryptu php:

 <?php
$host="localhost";
$username="root";
$password="password";
$db_name="baza";
 
$con=mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");
$sql = "select * from zastep";
$result = mysql_query($sql);
$json = array();
 
if(mysql_num_rows($result)){
while($row=mysql_fetch_assoc($result)){
$json['zastep'][]=$row;
}
}
mysql_close($con);
echo json_encode($json);
?> 

Skrypt działa poprawnie, błąd pokazaję się podczas kompilacji apki.

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.Menu;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.Toast;

import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;


public class MainActivity extends Activity{
    private String jsonResult;
    private String url = "http://192.168.2.100/tyr/conn.php";
    private ListView listView;
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        listView = (ListView) findViewById(R.id.listView);
        accessWebService();
    }
     @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }
    //uzyskiwanie dostępu do sieci
    private class JsonReadTask extends AsyncTask<String,Void, String> {
        @Override
        protected String doInBackground(String... params)
        {
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost(params[0]);
            try
            {
                HttpResponse response = httpclient.execute(httppost);
                jsonResult = inputStreamToString(response.getEntity().getContent()).toString();
            } catch (ClientProtocolException e) {
                e.printStackTrace();
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
            return null;
        }
        private StringBuilder inputStreamToString(InputStream is)
        {
            String rLine = "";
            StringBuilder answer = new StringBuilder();
            BufferedReader rd = new BufferedReader(new InputStreamReader(is));
            try {
                while ((rLine = rd.readLine())!= null)
                {
                    answer.append(rLine);
                }
            } catch (IOException e) {
                Toast.makeText(getApplicationContext(), "Error..." + e.toString(), Toast.LENGTH_LONG).show();
            }
            return answer;
        }
        @Override
        protected void onPostExecute(String result)
        {
            ListDrwaer();
        }
    }
    public void accessWebService()
    {
        JsonReadTask task = new JsonReadTask();
        task.execute(new String[] {url});
    }
    public void ListDrwaer()
    {
        List<Map<String, String>> employeeList = new ArrayList<Map<String, String>>();
        try
        {
            JSONObject jsonResponse = new JSONObject(jsonResult);
            JSONArray jsonMainNode = jsonResponse.optJSONArray("zastep");
            for (int i = 0; i < jsonMainNode.length(); i++)
            {
                JSONObject jsonChildNode = jsonMainNode.getJSONObject(i);
                String name = jsonChildNode.optString("klasa");
                String data = jsonChildNode.optString("data");
                String outPut = name + "-" + data;
                employeeList.add(createEmployee("employees", outPut));
            }
        }
        catch (JSONException e)
        {
            Toast.makeText(getApplicationContext(), "Error" + e.toString(),Toast.LENGTH_SHORT).show();
        }
        SimpleAdapter simpleAdapter = new SimpleAdapter(this, employeeList,android.R.layout.simple_list_item_1, new String[]{"employees"}, new int[] {android.R.id.text1});
        listView.setAdapter(simpleAdapter);
    }
    private HashMap<String, String> createEmployee(String name, String data)
    {
        HashMap<String, String> employeeNameNo = new HashMap<String,String>();
        employeeNameNo.put(name, data);
        return employeeNameNo;
    }

}

proszę o pomoc. :)