为什么它说它是一个字符串？

I am very very new in android app development and I need to get data from my local xampp mysql database. These are the error message I keep getting.

06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ org.json.JSONException: Value <br of type java.lang.String cannot be converted to JSONArray
06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at org.json.JSON.typeMismatch(JSON.java:111)
06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at org.json.JSONArray.<init>(JSONArray.java:96)
06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at org.json.JSONArray.<init>(JSONArray.java:108)
06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at com.example.shirlyn.attendancesystem.StudentJSONParser.parseFeed(StudentJSONParser.java:18)
06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at com.example.shirlyn.attendancesystem.MainActivity$ThreadClass.onPostExecute(MainActivity.java:111)
06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at com.example.shirlyn.attendancesystem.MainActivity$ThreadClass.onPostExecute(MainActivity.java:95)
06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at android.os.AsyncTask.finish(AsyncTask.java:636)
06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at android.os.AsyncTask.access$500(AsyncTask.java:177)
06-15 15:12:31.472  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:653)
06-15 15:12:31.473  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at android.os.Handler.dispatchMessage(Handler.java:102)
06-15 15:12:31.473  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at android.os.Looper.loop(Looper.java:135)
06-15 15:12:31.473  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at android.app.ActivityThread.main(ActivityThread.java:5254)
06-15 15:12:31.473  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at java.lang.reflect.Method.invoke(Native Method)
06-15 15:12:31.473  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at java.lang.reflect.Method.invoke(Method.java:372)
06-15 15:12:31.473  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
06-15 15:12:31.473  28252-28252/com.example.shirlyn.attendancesystem W/System.err﹕ at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)  

This is my php file get_all_products.php

<?php // array for JSON response
$response = array();

// include db connect class
require_once __DIR__ . '/db_connect.php';

// connecting to db
$db = new DB_CONNECT();

// get all products from products table
$result = mysql_query("SELECT * FROM student") or die(mysql_error());

// check for empty result
if (mysql_num_rows($result) > 0) {
// looping through all results
// products node
$response = array();

while ($row = mysql_fetch_assoc($result)) {
    // temp user array
    $response[] = $row;
}
// success

// echoing JSON response
echo json_encode($response);
} else {
// no products found
$response["success"] = 0;
$response["message"] = "No students found";

// echo no users JSON
header(‘Content-type: application/json’);
echo json_encode($response);
}
?>  

The results of get_all_products.php if I run it on my browser and it seems right...

[{"student_id":"TP028616","student_name":"Shirlyn Hoe Yee Lyn","student_username":"lyn","student_password":"123"},{"student_id":"TP033500","student_name":"Ryan Teh Hoon Meng","student_username":"teh","student_password":"123"}]

This is my java class in my android app StudentJSONParser.java

package com.example.shirlyn.attendancesystem;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.List;

public class StudentJSONParser {
    public static List<Student> parseFeed(String content){

        try {
            System.out.println("begin parsing");
            JSONArray ar = new JSONArray(content);
            List<Student> studentList = new ArrayList<>();

            for(int i = 0 ; i < ar.length() ; i++){
                System.out.println("loop " + i);
                JSONObject obj = ar.getJSONObject(i);
                Student student = new Student(obj.getString("student_id"), obj.getString("student_name"), obj.getString("student_username"), obj.getString("student_password"));

                studentList.add(student);
                System.out.println("complete loop " + i);
            }
            System.out.println("complete parsing");

            return studentList;

        } catch (JSONException e) {
            e.printStackTrace();
            System.out.println("error parsing");

            return null;
        }

    }
}

so is this the problem with the .php file not returning jsonarrays? or the logic of my app is wrong? Thanks for all the replies in advance.

this is my asynctask class

public class MainActivity extends ActionBarActivity implements View.OnClickListener {
private Button loginbtn;
private EditText usernameTxt, passwordTxt;
private ProgressBar pb;

private List<Student> studentslist;

public final static String readStudentphp = "http://192.168.0.103/webservice/get_all_products.php";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    OnClickButtonListener();
    initLogin();
    pb = (ProgressBar) findViewById(R.id.progressBar);
    pb.setVisibility(View.INVISIBLE);
}

private void initLogin(){
    usernameTxt = (EditText) findViewById(R.id.usernameTxt);
    passwordTxt = (EditText) findViewById(R.id.passwordTxt);
}

private void OnClickButtonListener(){
    loginbtn = (Button) findViewById(R.id.login);
    loginbtn.setOnClickListener(this);
}

private void requestData(String uri){
    ThreadClass t = new ThreadClass();
    t.execute(uri);
}

@Override
public void onClick(View v) {
    requestData(readStudentphp);
    //Intent intent = new Intent("com.example.shirlyn.attendancesystem.MainMenu");
    //startActivity(intent);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

private class ThreadClass extends AsyncTask<String, String, String>{
    @Override
    protected void onPreExecute(){
        pb.setVisibility(View.VISIBLE);
    }

    @Override
    protected String doInBackground(String... params) {
        String content = HTTPManager.getData(params[0]);

        return content;
    }

    @Override
    protected void onPostExecute(String result){
        System.out.println("Before parsing");
        studentslist = StudentJSONParser.parseFeed(result);
        System.out.println("After parsing");
        if(studentslist != null && studentslist.size() > 0)
            System.out.println(studentslist.get(0).getStudent_name());
        pb.setVisibility(View.INVISIBLE);
    }
}

}