错误org.json.JSONException

Question

I am creating an application for Android. It is the task of retrieving data from an external database server mysql. Everything was fine, but by the time of compilation. Gave me the error:

Errororg.json.JSONException: Value

A link to a database using php script:

$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);
?>

The script is working correctly, the error shows up when compiling apk.

 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;
    }

}

Logcat:

03-11 17:26:07.790  26957-26957/pl.saperdak1gmail.rrt E/﹕ appName=pl.saperdak1gmail.rrt, acAppName=/system/bin/surfaceflinger
03-11 17:26:07.790  26957-26957/pl.saperdak1gmail.rrt E/﹕ 0

please help