Android注册表不会提交(Volley和SQL）

我还是android编程的新手，目前正在学习如何连接到DB。我正在学习一个在线教程，但我仍然面临问题。本教程是关于创建一个注册表并通过截击将其发送到数据库的。我使用.hostinger.ae作为主机和Android工作室。一切似乎都很好，直到我点击注册按钮，它给了我一个错误（无法注册）我找不到错误，调试显示没有错误。所以请帮我找出问题所在。

我有两个php文件上传到公共文件夹上的在线主机

dbconnect php用于连接数据库的文件

<?php
define('HOST','mysql.hostinger.ae');
define('USER','--');
define('PASS','--');
define('DB','u853541076_tuber');
$con = mysqli_connect(HOST,USER,PASS,DB) or die('Unable to Connect');

和volleyRegister php

<?php
if($_SERVER['REQUEST_METHOD']=='POST'){
$username = $_POST['username'];    
$email = $_POST['email'];
$password = $_POST['password'];
require_once('dbConnect.php');
$sql = "INSERT INTO volley (username,password,email) VALUES('$username','$email','$password')";
  if(mysqli_query($con,$sql)){
  echo "Successfully Registered";
  }else{
  echo "Could not register";
  }
  }else{
 echo 'error';
 }

我的主要活动.javahttp://tuber.16mb.com是我的域

package myapp.com.tuberapp;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;
import org.json.JSONException;
import java.util.HashMap;
import java.util.Map;
public class MainActivity extends AppCompatActivity implements     View.OnClickListener {
///The hosting link
public static final String REGISTER_URL = "http://tuber.16mb.com/volleyRegister.php";
public static final String KEY_USERNAME = "username";
public static final String KEY_PASSWORD = "password";
public static final String KEY_EMAIL = "email";
private EditText editTextUsername;
private EditText editTextEmail;
private EditText editTextPassword;
private Button buttonRegister;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    editTextUsername = (EditText) findViewById(R.id.editTextUsername);
    editTextPassword = (EditText) findViewById(R.id.editTextPassword);
    editTextEmail= (EditText) findViewById(R.id.editTextEmail);
    buttonRegister = (Button) findViewById(R.id.buttonRegister);
    buttonRegister.setOnClickListener(this);
  }
  private void registerUser(){
    final String username = editTextUsername.getText().toString().trim();
    final String password = editTextPassword.getText().toString().trim();
    final String email = editTextEmail.getText().toString().trim();
    StringRequest stringRequest = new StringRequest(Request.Method.POST, REGISTER_URL,
            new Response.Listener<String>() {
                @Override
                public void onResponse(String response) {
                    Toast.makeText(MainActivity.this, response, Toast.LENGTH_LONG).show();
                }
            },
            new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    Toast.makeText(MainActivity.this,error.toString(),Toast.LENGTH_LONG).show();
                }
            }){
        @Override
        protected Map<String,String> getParams(){
            Map<String,String> params = new HashMap<String, String>();
            params.put(KEY_USERNAME,username);
            params.put(KEY_PASSWORD,password);
            params.put(KEY_EMAIL, email);
            return params;
        }
    };
    RequestQueue requestQueue = Volley.newRequestQueue(this);
    requestQueue.add(stringRequest);
   }
   @Override
  public void onClick(View v) {
    if(v == buttonRegister){
        registerUser();
    }
  }
  } 

manifest.java

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="myapp.com.tuberapp">

<uses-permission android:name="android.permission.INTERNET"/>
<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name"
        android:theme="@style/AppTheme.NoActionBar">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    </application>
    </manifest>