返回null的JSON对象（android+；JSON+；php+；mysql）

我有一张登记表，我正在做飞行验证。 在用户将焦点从email字段切换到off之后，我想检查数据库中是否注册了此类电子邮件，并为此干杯。 但是，无论我做什么，我总是在第585行得到一个空指针异常，这是下面的一行：

Log.d("Checking wether this email exists in database", json.toString());

事情是这样的：

class CheckWetherAUserWithSuchEmailExists extends AsyncTask<String, String, String> {

        int success;

        @Override
        protected String doInBackground(String... args) {

            int success;
            try {
                // Building Parameters
                List<NameValuePair> params = new ArrayList<NameValuePair>();
                Log.d("Kylie", userEmail);
                params.add(new BasicNameValuePair("user_email", userEmail));

                Log.d("request!", "starting");

                // Posting user data to script
                JSONObject json = jsonParser.makeHttpRequest(CHECK_EMAIL_EXISTS_URL, "POST", params);

                // full json response
                Log.d("Checking wether this email exists in database", json.toString());

                // json success element
                success = json.getInt(TAG_SUCCESS);
                if (success == 1) {
                    Log.d("Email exists", json.toString());
                    finish(); //fire up an alarm
                    userWithTheSameEmail = true;
                    return json.getString(TAG_MESSAGE);
                } else if(success == 2) {
                    Log.d("Email doesnt exist", json.getString(TAG_MESSAGE));
                    userWithTheSameEmail = false;
                    return json.getString(TAG_MESSAGE);
                } else {
                    Log.d("Something happened", json.getString(TAG_MESSAGE));
                    userWithTheSameEmail = false;
                    return json.getString(TAG_MESSAGE);
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }

            return null;

        }

这是错误堆栈跟踪：

 D/Kylie(4458): hdhdu@jfjd.com
 D/request!(4458): starting
Error parsing data org.json.JSONException: Value <br of type java.lang.String cannot be converted to JSONObject
 threadid=9: thread exiting with uncaught exception (group=0x4001e578)
FATAL EXCEPTION: AsyncTask #1
java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:200)
at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:274)
at java.util.concurrent.FutureTask.setException(FutureTask.java:125)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:308)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1088)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:581)
at java.lang.Thread.run(Thread.java:1019)
 Caused by: java.lang.NullPointerException
 net.asdasd.activities.Signup$CheckWetherAUserWithSuchEmailExists.doInBackground(Signup.java:585)
at net.asdasd.activities.Signup$CheckWetherAUserWithSuchEmailExists.doInBackground(Signup.java:1)
at android.os.AsyncTask$2.call(AsyncTask.java:185)
java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:306)
... 4 more

D/Kylie（4458）：hdhdu@jfjd.com
D/请求！（4458）：开始
解析数据org.json.JSONException时出错：Value据我所知，PHP代码中没有任何地方实际执行json输出。生成一个数组，但不使用它做任何事情

应该有一个

if (!empty($_POST)) {
   ... blah blah blah...
   echo json_encode($response);   // <<---this is missing, apparently
} else {
   echo json_encode('Something blew up');
}

if（！empty（$\u POST））{
……诸如此类。。。
echo json_encode（$response）；//是的，你是对的：））我从来没有真正学习过php，现在我正在努力学习。谢谢！
CREATE TABLE IF NOT EXISTS `users` (
  `user_id` int(11) NOT NULL AUTO_INCREMENT,
  `user_name` varchar(30) NOT NULL,
  `user_email` varchar(50) NOT NULL,
  `user_password` varchar(100) NOT NULL,
  `user_sex` varchar(10) NOT NULL,
  `user_avatar` varchar(100) DEFAULT NULL,
  `user_date_registered` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `user_last_seen` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `user_points` int(11) NOT NULL,
  PRIMARY KEY (`user_id`),
  UNIQUE KEY `user_name` (`user_name`,`user_email`),
  KEY `user_id` (`user_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=3 ;

 D/Kylie(4458): hdhdu@jfjd.com
 D/request!(4458): starting
Error parsing data org.json.JSONException: Value <br of type java.lang.String cannot be converted to JSONObject
 threadid=9: thread exiting with uncaught exception (group=0x4001e578)
FATAL EXCEPTION: AsyncTask #1
java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:200)
at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:274)
at java.util.concurrent.FutureTask.setException(FutureTask.java:125)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:308)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1088)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:581)
at java.lang.Thread.run(Thread.java:1019)
 Caused by: java.lang.NullPointerException
 net.asdasd.activities.Signup$CheckWetherAUserWithSuchEmailExists.doInBackground(Signup.java:585)
at net.asdasd.activities.Signup$CheckWetherAUserWithSuchEmailExists.doInBackground(Signup.java:1)
at android.os.AsyncTask$2.call(AsyncTask.java:185)
java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:306)
... 4 more

if (!empty($_POST)) {
   ... blah blah blah...
   echo json_encode($response);   // <<---this is missing, apparently
} else {
   echo json_encode('Something blew up');
}