android studio mysql php 연동회원가입

Question

public class LoginActivity extends AppCompatActivity {

    ViewFlipper Vf;
    Button loginButton, registerButton;
    EditText idText, passwordText;
    HttpPost httppost;
    StringBuffer buffer;
    HttpResponse response;
    HttpClient httpclient;
    List<NameValuePair> nameValuePairs;
    ProgressDialog dialog = null;
    TextView tv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
        StrictMode.setThreadPolicy(policy);

        loginButton = (Button)findViewById(R.id.loginButton);
        registerButton = (Button)findViewById(R.id.registerButton);
        idText = (EditText)findViewById(R.id.idText);
        passwordText = (EditText)findViewById(R.id.passwordText);
        loginButton.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                dialog = ProgressDialog.show(LoginActivity.this, "",
                        "Validating user...", true);
                new Thread(new Runnable() {
                    public void run() {
                        login();
                    }
                }).start();
            }
        });
    }
    void login() {
        try {
            httpclient = new DefaultHttpClient();
            httppost = new HttpPost("http://root/login.php");
            nameValuePairs = new ArrayList<NameValuePair>(2);
            nameValuePairs.add(new BasicNameValuePair("username", idText.getText().toString()));
            nameValuePairs.add(new BasicNameValuePair("password", passwordText.getText().toString()));
            httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
            response = httpclient.execute(httppost);
            ResponseHandler<String> responseHandler = new BasicResponseHandler();
            final String response = httpclient.execute(httppost, responseHandler);
            System.out.println("Response : " + response);
            runOnUiThread(new Runnable() {
                @Override
                public void run() {
                    tv.setText("Response from PHP : " + response);
                    dialog.dismiss();
                }
            });
            if (response.equalsIgnoreCase("User Found")) {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(LoginActivity.this, "Login Success", Toast.LENGTH_SHORT).show();

                    }
                });
                // 로그인 성공시 MainActivity 전환
                startActivity((new Intent(LoginActivity.this, MainActivity.class)));
                finish();
            } else {
                Toast.makeText(LoginActivity.this, "Login Fail", Toast.LENGTH_SHORT).show();
            }
        }
        catch(Exception e)
        {
            dialog.dismiss();
            System.out.println("Exception : " + e.getMessage());
        }
    }

    public void Register(View view)
    {
        Intent intent = new Intent(this, RegisterActivity.class);
        startActivity(intent);
    }
}

login.php

<?php
  $connect = @mysql_connect("localhost","root","apmsetup") or die("error");
  $dbname = "mydb";
  $dbconn = mysql_select_db($dbname,$connect);
 $username = $_POST['username'];

    $query_search = "select * from custom_info where u_id = '".$username."' AND u_pw = '".$password. "'";
    $password = $_POST['password'];
    $query_exec = mysql_query($query_search) or die(mysql_error());
        echo "User Found";
    $rows = mysql_num_rows($query_exec);

    if($rows == 0) {
        echo "No Such User Found";
    }
    else  {
    }
?>

DB에는 잘 들어가는데...

여기서 회원가입후 로그인하면 액티비티가 RegisterAcitivty.java 로 넘어가는데

액티비티 전환 소스 제대로 했는데 왜 회원가입창으로 다시 넘어가나요ㅠㅠ?

회원가입할때 만약 DB에 있는 id 와 회원가입하려는 id 값이 같으면 이미 있다고 창 뜨는 거랑

로그인시 입력한 아이디와 패스워드 DB에 없으면 로그인 실패라는 창 띄우려는데 어떻게 써야하죠..?

더 좋은 회원가입 소스 있으면 url 남겨주시면 감사하겠습니다..