JSON从PHP返回null

Question

Json总是从PHP服务器返回null，但是，我的登录，重置和更改密码返回一个值。任何人都可以请帮助并告诉我如何找出JSON从PHP服务器返回null值的原因。

数据进入数据库，然而，当JSON从php服务器返回null值时，应用程序崩溃。我从Android端进行了调试，可以看到JSON总是返回一个空值，但是，这些字段确实有值，数据会进入数据库。

index.php

<?php

/*
 PHP API for Login, Register Requests and for Email Notifications.
 */

require_once 'mailer.php';

if (isset($_POST['tag']) && $_POST['tag'] != '')
{
    // Include Database handler
    require_once 'include/DB_Functions.php';
    $db = new DB_Functions();

    // response Array
    $response = array("tag" => $_POST['tag'], "success" => 0, "error" => 0);

    // check for tag type
    switch($_POST['tag'])
    {
        case 'register':
            // Request type is Register new user
            $fname = $_POST['fname'];
            $lname = $_POST['lname'];
            $email = $_POST['email'];
            $uname = $_POST['uname'];
            $password = $_POST['password'];

            // check if user is already existed
            if ($db->isUserExisted($email))
            {
                // user is already existed - error response
                $response["error"] = 2;
                $response["error_msg"] = "User already existed";
            } 
            else if(!$db->validEmail($email))
            {
                $response["error"] = 3;
                $response["error_msg"] = "Invalid Email Id";          
            }
            else
            {
                // store user
                $user = $db->storeUser($fname, $lname, $email, $uname, $password);
                if ($user)
                {
                    // user stored successfully
                    $response["success"] = 1;
                    $response["user"]["fname"] = $user["firstname"];
                    $response["user"]["lname"] = $user["lastname"];
                    $response["user"]["email"] = $user["email"];
                    $response["user"]["uname"] = $user["username"];
                    $response["user"]["uid"] = $user["unique_id"];
                    $response["user"]["created_at"] = $user["created_at"];

                    $subject = "Registration";
                    $message = "Hello $fname,\n\nYou have successfully registered to our service.\n\nRegards,\nAdmin.";

                    $name = $user['firstname'] . ' ' . $user['lastname'];
                    send_email($subject, $message, $email, $name);
                }
                else
                {
                    // user failed to store
                    $response["error"] = 1;
                    $response["error_msg"] = "JSON Error occured in Registartion";
                }
            }
        break;

        default:
            $response["error"] = 3;
            $response["error_msg"] = "JSON ERROR";
    }

    echo json_encode($response);
}
else
{
    echo "BradVisor Login API";
}
?>

DB_Functions.PHP

/**
     * Adding new user to mysqli database
     * returns user details
     */

    public function storeUser($fname, $lname, $email, $uname, $password) {
        $uuid = uniqid('', true);
        $hash = $this->hashSSHA($password);
        $encrypted_password = $hash["encrypted"]; // encrypted password
        $salt = $hash["salt"]; // salt
        $result = mysqli_query($this->db,"INSERT INTO `users`(`unique_id`, `firstname`, `lastname`, `username`, `email`, `encrypted_password`, `salt`, `created_at`) VALUES('$uuid', '$fname', '$lname', '$uname', '$email', '$encrypted_password', '$salt', NOW())") or die(mysqli_error($db)); 

  // check for successful store
        if ($result) {
            // get user details 
            $uid = mysqli_insert_id($this->db); // last inserted id
            $result = mysqli_query($this->db, "SELECT * FROM users WHERE uid = $uid") or die(mysqli_error($this->db));
            // return user details
            return mysqli_fetch_array($result);
        } else {
            return false;
        }
    }