java - data not retrieving from online mysql server

I am trying to get data from online database(MySql) through json and want to display it as a ListView. What I've done so far :

Downloader.java

public class Downloader extends AsyncTask {
Context c;
String address;
ListView lv;
ProgressDialog pd;
public Downloader(Context c, String address, ListView lv) {
    this.c = c;

    this.address = address;
    this.lv = lv;
}
//B4 JOB STARTS
@Override
protected void onPreExecute() {
    super.onPreExecute();
    pd=new ProgressDialog(c);
    pd.setTitle("Fetch Data");
    pd.setMessage("Fetching Data...Please wait");

    pd.show();
}
@Override
protected String doInBackground(Void... params) {
    String data=downloadData();
    return data;
}
@Override
protected void onPostExecute(String s) {
    super.onPostExecute(s);

    pd.dismiss();;
    if(s != null)
    {
        Parser p=new Parser(c,s,lv);
        p.execute();
    }else
    {
        Toast.makeText(c,"Unable to download data",Toast.LENGTH_SHORT).show();
    }
}

private String downloadData()
{
    //connect and get a stream
    InputStream is=null;
    String line =null;
    try {
        URL url=new URL(address);
        HttpURLConnection con= (HttpURLConnection) url.openConnection();
        is=new BufferedInputStream(con.getInputStream());
        BufferedReader br=new BufferedReader(new InputStreamReader(is));

        StringBuffer sb=new StringBuffer();
        if(br != null) {
            while ((line=br.readLine()) != null) {
                sb.append(line+"n");
            }
        }else {
            return null;
        }
        return sb.toString();
        } catch (MalformedURLException e) {

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

            }
        }
    }
return null;
}

}

MainActivity.java

public class MainActivity extends Activity {
 String url="http://bookvilla.esy.es/book.php";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final ListView lv= (ListView) findViewById(R.id.lv);
        final Downloader d=new Downloader(this,url,lv);

        d.execute();

    }
}

when i run the app the app.. i get a toast message saying "unable to download data" which is in the 35th line of Downloader.java class. So i am guessing that the data i am getting from the database is null ...but why is it so ....

$con = mysqli_connect($host,$u_name,$pwd,$db) or die('Unable to connect'); if(mysqli_connect_error($con)) { echo "Failed to connect to
database".mysqli_connect_error();

}

$query = mysqli_query($con,"SELECT * FROM playerstb"); if($query) {

while($row=mysqli_fetch_array($query)) { $flag[] = $row;

} print(json_encode($flag)); } mysqli_close($con);

above is my php file