JSONException for android connection with mysql
===

i have an android application which should enable to login different users using a pho file and mysql database but i the connection not established because of JSON error:
php file:

    <?php
    $dbhost="localhost";
    $dbuser="";
    $dbpass="";
    $dbdb="test";
    $connect = mysql_connect($dbhost,$dbuser,$dbpass)or die("connection error");
    mysql_select_db($dbdb)or die ("database selection error");
    $username=isset($_POST['username']);
    $password=isset($_POST['password']);
    $query = mysql_query("SELECT * FROM table1 WHERE username = '$username' AND password = '$password'");
    $num = mysql_num_rows($query);
    if($num == 1) { while($list = mysql_fetch_assoc($query)){
    $output=$list;
    echo json_encode($output);
    }
    mysql_close();}?>
android code:

    ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
	    nameValuePairs.add(new BasicNameValuePair("username",username));
		nameValuePairs.add(new BasicNameValuePair("password",password));
		try {
			httpclient = new DefaultHttpClient() ;
			httppost = new HttpPost("http://10.0.2.2/projet/connection.php");
			httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
			response = httpclient.execute(httppost);
			Log.i("connect", response.getStatusLine().toString());
			//verifier requete http de code 200
			if(response.getStatusLine().getStatusCode()==200){
				entity = response.getEntity();
				//verifier que entity non null
				if(entity != null){
					InputStream instream = entity.getContent();
					//creer JSON object ayant converted data comme parametre
					JSONObject jsonresponse = new JSONObject(convertStreamToString(instream));
					//affecter json response à une variable locale 
					String retUser = jsonresponse.getString("username");
					String retpass = jsonresponse.getString("password");
					//valider login
					if (username.equals(retUser) && password.equals(retpass)){
						Log.i("connect", response.getStatusLine().toString());
						//creation de SharedPreference pour enregistrer les login details
						SharedPreferences sp = getSharedPreferences("logindetails",0);
						//modifier sharedPreferences
						SharedPreferences.Editor spedit = sp.edit();
						//mettre logindetails comme string
						spedit.putString("username", username);
						spedit.putString("password", password);
						//fermer editor
						spedit.commit();
						appelle();
						Toast.makeText(getBaseContext(), "connexion avec succés", Toast.LENGTH_SHORT).show();
						
					}else {
						Toast.makeText(getBaseContext(), "login details invalide", Toast.LENGTH_SHORT).show();
					}
				}
			}
		}catch(Exception e){
			e.printStackTrace();
			Toast.makeText(getBaseContext(), "Erreur lors du connexion", Toast.LENGTH_SHORT).show();
		}	
	}
	
	 private static String convertStreamToString(InputStream is) {
		 //méthode pour parcourir la table et lire les données
	        /*
	         * To convert the InputStream to String we use the BufferedReader.readLine()
	         * method. We iterate until the BufferedReader return null which means
	         * there's no more data to read. Each line will appended to a StringBuilder
	         * and returned as String.
	         */
	        BufferedReader reader = new BufferedReader(new InputStreamReader(is));
	        StringBuilder sb = new StringBuilder();
	 
	        String line = null;
	        try {
	            while ((line = reader.readLine()) != null) {
	                sb.append(line + "\n");
	            }
	        } catch (IOException e) {
	            e.printStackTrace();
	        } finally {
	            try {
	                is.close();
	            } catch (IOException e) {
	                e.printStackTrace();
	            }
	        }
	        return sb.toString();
	    }
JSONException:

    07-31 11:03:36.995: W/System.err(342): org.json.JSONException: End of input at character 0 of 
help me please thx