private class RegistrationTask extends AsyncTask<Void, Void, Boolean> {
    private final String TAG = "RegistrationTask: ";
    private final String email;
    private final String password;
    private boolean success;
    private StringBuffer response;

    RegistrationTask(String email, String password) {
        this.email = email;
        this.password = password;
    }

    @Override
    protected Boolean doInBackground(Void... params) {
        try {
            Map<String, String> paramdata = new HashMap<>();
            paramdata.put("tag", "register");
            paramdata.put("email", email);
            paramdata.put("password", password);

            String registerUrl = "http://marshalllee.netne.net/LoginModule/index.php";
            URL url = new URL(registerUrl);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setReadTimeout(10000);
            connection.setConnectTimeout(15000);
            connection.setRequestMethod("POST");
            connection.setDoOutput(true);

            StringBuffer requestParams = new StringBuffer();

            if(paramdata != null && paramdata.size() > 0) {
                Iterator<String> paramIterator = paramdata.keySet().iterator();
                while(paramIterator.hasNext()) {
                    String key = paramIterator.next();
                    String value = paramdata.get(key);
                    requestParams.append(URLEncoder.encode(key, "UTF-8"));
                    requestParams.append("=").append(URLEncoder.encode(value, "UTF-8"));
                    requestParams.append("&");
                }

                // send POST request
                OutputStreamWriter writer = new OutputStreamWriter(connection.getOutputStream());
                writer.write(requestParams.toString());
                Log.v(TAG, requestParams.toString());
                writer.flush();
            }

            // get the response from the server.

            BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String line = null;
            response = new StringBuffer();
            while((line = in.readLine()) != null) {
                response.append(line);

            }

            success = true;

        } catch(IOException e) {
            Log.v(TAG, e.getMessage());
            success = false;
        }
        return success;
    }

    @Override
    protected void onPostExecute(Boolean aBoolean) {
        super.onPostExecute(aBoolean);
        if(aBoolean) {
            Log.v(TAG, "success");
        } else {
            Log.v(TAG, "failure");
        }
    }

}

01-11 21:26:53.336 6655-7021/? V/RegistrationTask:: Response: {"error":true,"error_msg":"User already exists"}string(4) "POST"array(3) {  ["password"]=>  string(8) "********"  ["email"]=>  string(24) "********@gmail.com"  ["tag"]=>  string(8) "register"}<!-- Hosting24 Analytics Code --><script type="text/javascript" src="http://stats.hosting24.com/count.php"></script><!-- End Of Analytics Code -->

This is the corresponding PHP code.

<?php

require_once 'db/db_functions.php';
$db = new DB_Functions();

if (isset($_POST['tag']) && !empty($_POST['tag'])) {

    $tag = $_POST['tag'];

    // get the variables.
    $email = $_POST['email'];
    $password = $_POST['password'];

    // response Array
    $response = array("error" => FALSE);

    // check for tag type
    if ($tag == 'login') {

        // check for user
        $user = $db->getUserByEmailAndPassword($email, $password);
        if ($user != false) {

            // user found
            $response["error"] = FALSE;
            $response["uid"] = $user["unique_id"];
            $response["user"]["email"] = $user["email"];
            $response["user"]["created_at"] = $user["created_at"];
            $response["user"]["updated_at"] = $user["updated_at"];

            echo json_encode($response);
        } else {
            // user not found
            // echo json with error = 1
            $response["error"] = TRUE;
            $response["error_msg"] = "Incorrect email or password!";
            echo json_encode($response);
        }
    } else if ($tag == 'register') {

        // check if user is already exists

        if ($db->userExists($email)) {

            // user already exists - error response
            $response["error"] = TRUE;
            $response["error_msg"] = "User already exists";
            echo json_encode($response);

        } else {

            // store user
            $user = $db->storeUser($email, $password);


            if ($user != false) {
                // user successfully saved to MySQL database
                $response["error"] = FALSE;
                $response["uid"] = $user["unique_id"];
                $response["user"]["email"] = $user["email"];
                $response["user"]["created_at"] = $user["created_at"];
                $response["user"]["updated_at"] = $user["updated_at"];
                echo json_encode($response);

            } else {
                // user failed to store
                $response["error"] = TRUE;
                $response["error_msg"] = "JSON Error occured in Registration";
                echo json_encode($response);
            }
        }
    } else {
        // user failed to store
        $response["error"] = TRUE;
        $response["error_msg"] = "Unknow 'tag' value. It should be either 'login' or 'register'";
        echo json_encode($response);
    }
} else {

    $response["error"] = TRUE;
    $response["error_msg"] = "Operation failed due to the missing tag!";
    echo json_encode($response);
}


var_dump($_SERVER['REQUEST_METHOD'], $_POST);
?>