Trying to run query to get user_id based on logged in username. 
users.php....
<?php
session_start();
include_once 'error.php';
class User{
private $db;
private $db_table = "users";
public function __construct()
{
    $this->db = new DbConnect();
}
public function isLoginExist($username, $password)
{       
    $query = "select * from " . $this->db_table . " where username =     
 '$username' AND password = '$password' Limit 1";
    $result = mysqli_query($this->db->getDb(), $query);
    if(mysqli_num_rows($result) > 0){
        mysqli_close($this->db->getDb());
        return true;
    }       
    mysqli_close($this->db->getDb());
    return false;       
}
public function createNewRegisterUser($username, $password, $email)
{
    $query = "insert into users (username, password, email, created_at, 
updated_at) values ('$username', '$password', '$email', NOW(), NOW())";
    $inserted = mysqli_query($this->db->getDb(), $query);
    if($inserted == 1){
        $json['success'] = 1;                                   
    }else{
        $json['success'] = 0;
    }
    mysqli_close($this->db->getDb());
    return $json;
}
public function loginUsers($username, $password){
    $json = array();
    $canUserLogin = $this->isLoginExist($username, $password);
    if($canUserLogin){
        $json['success'] = 1;
    }else{
        $json['success'] = 0;
    }
    return $json;
}
}
?>
index.php
<?php
session_start();
require_once 'users.php';
$username = "";
$password = "";
$email = "";
if(isset($_POST['username'])){
$username = $_POST['username'];
}
if(isset($_POST['password'])){
$password = $_POST['password'];
}
if(isset($_POST['email'])){
$email = $_POST['email'];
}
// Instance of a User class
$userObject = new User();
// Registration of new user
if(!empty($username) && !empty($password) && !empty($email)){
$hashed_password = md5($password);
$json_registration = $userObject->createNewRegisterUser($username,     
$hashed_password, $email);
echo json_encode($json_registration);
}
// User Login
if(!empty($username) && !empty($password))
{
$hashed_password = md5($password);  
$json_array = $userObject->loginUsers($username, $hashed_password);
session_start();
$_SESSION['username'] = $username;
echo json_encode($json_array);
}
//var_dump($_SESSION['username']);displays current users name on android LOG
?>
topics.php
<?php
session_start();
include_once 'error.php';
class Topic{
private $db;
private $db_table = "topics";
private $db_table1 = "created_topics";
public function __construct()
{
    $this->db = new DbConnect();
}
public function createNewTopic($topic_name, $content)
{   
    session_start();
    include_once 'index.php';   
    //query to get current logged in user_id
    $un = "SELECT user_id FROM users WHERE username = " .    
    $_SESSION['username'] . " LIMIT 1";
    //running query
    $unResults = mysqli_query($this->db->getDb(), $un);
    //insert into db topic_name and content
    $query = "INSERT INTO topics (topic_name, content, created_at, 
    updated_at) values ('$topic_name', '$content', NOW(), NOW())";
    $inserted = mysqli_query($this->db->getDb(), $query);
    //query to insert into created_topics table with  user_id and topic_id
    $q = "insert into created_topics(user_id, topic_id,created_at) values 
    ('$unResults',LAST_INSERT_ID(),NOW())";
    mysqli_query($this->db->getDb(), $q);
    if($inserted == 1){
        $json['success'] = 1;                                   
    }else{
        $json['success'] = 0;
    }
    mysqli_close($this->db->getDb());
    return $json;
}
}
?>
created_topic.php
<?php
session_start();
require_once 'topics.php';
$topic_name = "";
$content = "";
$username = $_SESSION['username'];
if(isset($_POST['topic_name']))
{
$topic_name = $_POST['topic_name'];
}
if(isset($_POST['content']))
{
$content = $_POST['content'];
}
// Instance of a Topic class
$topicObject = new Topic();
// Registration of new topic
if(!empty($topic_name) && !empty($content))
{
$json_registration = $topicObject->createNewTopic($topic_name, $content);
echo json_encode($json_registration);
}
?>
Android create_topic page
package com.example.mrbuknahsty.annovoteexdb;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.params.BasicHttpParams;
import org.apache.http.params.HttpConnectionParams;
import org.apache.http.params.HttpParams;
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.List;
public class createTopic extends AppCompatActivity
{
 protected EditText enteredTopicName,enteredContent;
Button  create;
protected String topic_name;
private final String serverUrl1 =      
 "http://lkirkpatrick.btcwsd.com/anno/create_topic.php";
@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_create_topic);
    enteredTopicName = (EditText) findViewById(R.id.topicNameET);
    enteredContent = (EditText) findViewById(R.id.contentEdit);
    create = (Button)findViewById(R.id.createBtn);
    create.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            topic_name = enteredTopicName.getText().toString();
            String content = enteredContent.getText().toString();
            if(topic_name.equals("") || content.equals("")){
                Toast.makeText(createTopic.this, "Topic Name or Content must      
          be filled", Toast.LENGTH_LONG).show();
                return;
            }
            if(topic_name.length() <= 1 || content.length() <= 1){
                Toast.makeText(createTopic.this, "Topic Name or Content     
        length must be greater than one", Toast.LENGTH_LONG).show();
                return;
            }
    // request authentication with remote server4
            AsyncDataClass asyncRequestObject = new AsyncDataClass();
            asyncRequestObject.execute(serverUrl1, topic_name, content);
        }
    });
}
private class AsyncDataClass extends AsyncTask<String, Void, String> {
    @Override
    protected String doInBackground(String... params) {
        HttpParams httpParameters = new BasicHttpParams();
        HttpConnectionParams.setConnectionTimeout(httpParameters, 5000);
        HttpConnectionParams.setSoTimeout(httpParameters, 5000);
        HttpClient httpClient = new DefaultHttpClient(httpParameters);
        HttpPost httpPost = new HttpPost(params[0]);
        String jsonResult = "";
        try {
            List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>   
            (2);
            nameValuePairs.add(new BasicNameValuePair("topic_name", 
           params[1]));
            nameValuePairs.add(new BasicNameValuePair("content", params[2]));
            nameValuePairs.add(new BasicNameValuePair("content", params[2]));
            httpPost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
            HttpResponse response = httpClient.execute(httpPost);
            jsonResult =     
     inputStreamToString(response.getEntity().getContent()).toString();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return jsonResult;
    }
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }
    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        System.out.println("Resulted Value: " + result);
        if(result.equals("") || result == null){
            Toast.makeText(createTopic.this, "Server connection failed", 
         Toast.LENGTH_LONG).show();
            return;
        }
        int jsonResult = returnParsedJsonObject(result);
        if(jsonResult == 0){
            Toast.makeText(createTopic.this, "Something Went Wrong", 
         Toast.LENGTH_LONG).show();          
            return;
        }
        if(jsonResult == 1){
            Intent intent = new Intent(createTopic.this, login.class);
            intent.putExtra("USERNAME", topic_name);
            intent.putExtra("MESSAGE", "Topic successfully created!");
            startActivity(intent);
        }
    }
    private StringBuilder inputStreamToString(InputStream is) {
        String rLine = "";
        StringBuilder answer = new StringBuilder();
        BufferedReader br = new BufferedReader(new InputStreamReader(is));
        try {
            while ((rLine = br.readLine()) != null) {
                answer.append(rLine);
            }
        } catch (IOException e) {
// TODO Auto-generated catch block
            e.printStackTrace();
        }
        return answer;
    }
}
private int returnParsedJsonObject(String result){
    JSONObject resultObject = null;
    int returnedResult = 0;
    try {
        resultObject = new JSONObject(result);
        returnedResult = resultObject.getInt("success");
    } catch (JSONException e) {
        e.printStackTrace();
    }
    return returnedResult;
}
}
Everything ran fine until i added the query to get user_id from users where username = '$username';
Now in my android log i only get-- Resulted value:
and my toast says server connection fail. Any help would be awesome.
Thanks
 
    