急！新手求助！关于Android调用webservice出错！！

我是完全根据这个教程复制的代码：
http://www.androidhive.info/2012/01/android-login-and-registration-with-php-mysql-and-sqlite/然后把mysql的用户密码什么的都改成我自己的了。但还是出现了错误。下面是其中一个出错的activity的代码：package com.example.androidhive;
 
import org.json.JSONException;
import org.json.JSONObject;
 
import com.example.androidhive.library.DatabaseHandler;
import com.example.androidhive.library.UserFunctions;
 
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
 
public class RegisterActivity extends Activity {
    Button btnRegister;
    Button btnLinkToLogin;
    EditText inputFullName;
    EditText inputEmail;
    EditText inputPassword;
    TextView registerErrorMsg;
     
    // JSON Response node names
    private static String KEY_SUCCESS = "success";
    private static String KEY_ERROR = "error";
    private static String KEY_ERROR_MSG = "error_msg";
    private static String KEY_UID = "uid";
    private static String KEY_NAME = "name";
    private static String KEY_EMAIL = "email";
    private static String KEY_CREATED_AT = "created_at";
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.register);
 
        // Importing all assets like buttons, text fields
        inputFullName = (EditText) findViewById(R.id.registerName);
        inputEmail = (EditText) findViewById(R.id.registerEmail);
        inputPassword = (EditText) findViewById(R.id.registerPassword);
        btnRegister = (Button) findViewById(R.id.btnRegister);
        btnLinkToLogin = (Button) findViewById(R.id.btnLinkToLoginScreen);
        registerErrorMsg = (TextView) findViewById(R.id.register_error);
         
        // Register Button Click event
        btnRegister.setOnClickListener(new View.OnClickListener() {         
            public void onClick(View view) {
                String name = inputFullName.getText().toString();
                String email = inputEmail.getText().toString();
                String password = inputPassword.getText().toString();
                UserFunctions userFunction = new UserFunctions();
                JSONObject json = userFunction.registerUser(name, email, password);
                 
                // check for login response
                try {
                    if (json.getString(KEY_SUCCESS) != null && json.getString(KEY_SUCCESS)!="") {
                        registerErrorMsg.setText("");
                        String res = json.getString(KEY_SUCCESS); 
                        if(Integer.parseInt(res) == 1){
                            // user successfully registred
                            // Store user details in SQLite Database
                            DatabaseHandler db = new DatabaseHandler(getApplicationContext());
                            JSONObject json_user = json.getJSONObject("user");
                             
                            // Clear all previous data in database
                            userFunction.logoutUser(getApplicationContext());
                            db.addUser(json_user.getString(KEY_NAME), json_user.getString(KEY_EMAIL), json.getString(KEY_UID), json_user.getString(KEY_CREATED_AT));                        
                            // Launch Dashboard Screen
                            Intent dashboard = new Intent(getApplicationContext(), DashboardActivity.class);
                            // Close all views before launching Dashboard
                            dashboard.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                            startActivity(dashboard);
                            // Close Registration Screen
                            finish();
                        }else{
                            // Error in registration
                            registerErrorMsg.setText("Error occured in registration");
                        }
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }
        });
 
        // Link to Login Screen
        btnLinkToLogin.setOnClickListener(new View.OnClickListener() {
 
            public void onClick(View view) {
                Intent i = new Intent(getApplicationContext(),
                        LoginActivity.class);
                startActivity(i);
                // Close Registration View
                finish();
            }
        });
    }
}
然后下面是logcat：07-08 12:50:08.025: E/JSON(655): <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">n<html><head>n<title>404 Not Found</title>n</head><body>n<h1>Not Found</h1>n<p>The requested URL /ah_login_api/ was not found on this server.</p>n</body></html>n
07-08 12:50:08.025: E/JSON Parser(655): Error parsing data org.json.JSONException: Value <!DOCTYPE of type java.lang.String cannot be converted to JSONObject
07-08 12:50:08.035: D/AndroidRuntime(655): Shutting down VM
07-08 12:50:08.035: W/dalvikvm(655): threadid=1: thread exiting with uncaught exception (group=0x40015560)
07-08 12:50:08.045: E/AndroidRuntime(655): FATAL EXCEPTION: main
07-08 12:50:08.045: E/AndroidRuntime(655): java.lang.NullPointerException
07-08 12:50:08.045: E/AndroidRuntime(655):  at com.example.androidhive.RegisterActivity$1.onClick(RegisterActivity.java:59)
07-08 12:50:08.045: E/AndroidRuntime(655):  at android.view.View.performClick(View.java:2485)
07-08 12:50:08.045: E/AndroidRuntime(655):  at android.view.View$PerformClick.run(View.java:9080)
07-08 12:50:08.045: E/AndroidRuntime(655):  at android.os.Handler.handleCallback(Handler.java:587)
07-08 12:50:08.045: E/AndroidRuntime(655):  at android.os.Handler.dispatchMessage(Handler.java:92)
07-08 12:50:08.045: E/AndroidRuntime(655):  at android.os.Looper.loop(Looper.java:123)
07-08 12:50:08.045: E/AndroidRuntime(655):  at android.app.ActivityThread.main(ActivityThread.java:3683)
07-08 12:50:08.045: E/AndroidRuntime(655):  at java.lang.reflect.Method.invokeNative(Native Method)
07-08 12:50:08.045: E/AndroidRuntime(655):  at java.lang.reflect.Method.invoke(Method.java:507)
07-08 12:50:08.045: E/AndroidRuntime(655):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
07-08 12:50:08.045: E/AndroidRuntime(655):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
07-08 12:50:08.045: E/AndroidRuntime(655):  at dalvik.system.NativeStart.main(Native Method)
求助到底哪里出错了？？AndroidWeb服务JSON