未捕获的错误:使用php脚本调用未定义的方法mysqli_stmt :: get_result()问题

I went through all stakoverflow posts about the problem, and everywhere I find only some other procedure how to solve it. I am trying to store data to the database through android application, I had been doing that successfully until yesterday... Then I made some changes in the database and suddenly I cannot use my PHP anymore... Maybe the problem was that I added to column to database? Please help me if you can.

Here are my php files: DB_Connect.php:

<?php
class DB_Connect {
    private $conn;

    public function connect() {
        require __DIR__ . "/Config.php";

        $this->conn = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_DATABASE);
        return $this->conn;
    }
}

?>

DB_Functions.php:

 <?php

    class DB_Functions {

        private $conn;

        function __construct() {
            require __DIR__ . "/DB_Connect.php";
            $db = new Db_Connect();
            $this->conn = $db->connect();
        }

        // destructor
        function __destruct() {

        }

        /**
         * Storing new user
         * returns user details
         */
        public function storeUser($name, $email, $password, $facebook_json) {
            $uuid = uniqid('', true);
            $hash = $this->hashSSHA($password);
            $encrypted_password = $hash["encrypted"]; // encrypted password
            $salt = $hash["salt"]; // salt

            $stmt = $this->conn->prepare("INSERT INTO users(unique_id, name, email, encrypted_password, salt, facebook_json, created_at) VALUES(?, ?, ?, ?, ?, ?, NOW())");
            $stmt->bind_param("ssssss", $uuid, $name, $email, $encrypted_password, $salt, $facebook_json);
            $result = $stmt->execute();
            $stmt->close();

            // check for successful store
            if ($result) {
                $stmt = $this->conn->prepare("SELECT * FROM users WHERE email = ?");
                $stmt->bind_param("s", $email);
                $stmt->execute();
                $user = $stmt->get_result()->fetch_assoc();
                $stmt->close();

                return $user;
            } else {
                return false;
            }
        }

         /**
         * Get user by email
         */
        public function getUserByEmail($email) {
            $stmt = $this->conn->prepare("SELECT * FROM `users` WHERE `email` = ? ");

            $stmt->bind_param("s", $email);

            if ($stmt->execute()) {
                $user = $stmt->get_result()->fetch_assoc();
                $stmt->close();

                return $user;
            } else {
                return NULL;
            }
        }

        /**
         * Check user is existed or not
         */
        public function isUserExisted($email) {
            $stmt = $this->conn->prepare("SELECT email from users WHERE email = ?");
            $stmt->bind_param("s", $email);

            $stmt->execute();

            $stmt->store_result();

            if ($stmt->num_rows > 0) {
                // user existed 
                $stmt->close();
                return true;
            } else {
                // user not existed
                $stmt->close();
                return false;
            }
        }
        /**
         * Updating facebook_json field
         */
        public function updateFacebookJson($email, $facebook_json) {
            $stmt = $this->conn->prepare("UPDATE users SET facebook_json = ?, updated_at = NOW() WHERE email = ?");

            $stmt->bind_param("ss",$facebook_json, $email);

            $stmt->execute();

            $stmt->store_result();
        }

         /**
         * Updating password for facebook_login first time
         */
        public function updatePasswordFacebook($email, $password) {
            $stmt = $this->conn->prepare("SELECT * FROM users WHERE email = ?");

            $stmt->bind_param("s", $email);

            if ($stmt->execute()) {
                $user = $stmt->get_result()->fetch_assoc();
                $stmt->close();
                $hash = $this->hashSSHA($password);
                $encrypted_password = $hash["encrypted"];
                $salt = $hash["salt"];

                $stmt = $this->conn->prepare("UPDATE users SET encrypted_password=?, salt=?, updated_at = NOW() WHERE email = ?");

                $stmt->bind_param("sss",$encrypted_password, $salt, $email);

                $result = $stmt->execute();

                $stmt->store_result();
                $stmt->close();
                if($result) {
                    return true;
                } else {
                    return false;
                }
            } else {
                return false;
            }
        }

        /**
         * Encrypting password
         * @param password
         * returns salt and encrypted password
         */
        public function hashSSHA($password) {

            $salt = sha1(rand());
            $salt = substr($salt, 0, 10);
            $encrypted = base64_encode(sha1($password . $salt, true) . $salt);
            $hash = array("salt" => $salt, "encrypted" => $encrypted);
            return $hash;
        }

        /**
         * Decrypting password
         * @param salt, password
         * returns hash string
         */
        public function checkhashSSHA($salt, $password) {

            $hash = base64_encode(sha1($password . $salt, true) . $salt);

            return $hash;
        }

    }

    ?>

register.php:

 <?php
    require_once  __DIR__ .'/include/DB_Functions.php';

    $db = new DB_Functions();

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

    if (isset($_POST['name']) && isset($_POST['email']) && isset($_POST['password'])) {

        // receiving the post params
        $name = $_POST['name'];
        $email = $_POST['email'];
        $password = $_POST['password'];
        $facebook_json = NULL;
        if(isset($_POST['facebook_json'])) {
            $facebook_json = $_POST['facebook_json'];
        }


        // check if user is already existed with the same email
        if ($db->isUserExisted($email)) {
            // user already existed
            if(!($facebook_json === NULL)) {
                $db->updateFacebookJson($email, $facebook_json);
            }
            $user = $db->getUserByEmail($email);
            $response["error"] = TRUE;
            $response["error_msg"] = "User already existed with " . $email;
            $response["user"]["name"] = $user["name"];
            $response["user"]["email"] = $user["email"];
            $response["user"]["facebook_json"] = $user["facebook_json"];
            $response["user"]["created_at"] = $user["created_at"];
            $response["user"]["updated_at"] = $user["updated_at"];
            echo json_encode($response);
        } else {
            // create a new user
            $user = $db->storeUser($name, $email, $password, $facebook_json);
            if ($user) {
                // user stored successfully
                $response["error"] = FALSE;
                $response["uid"] = $user["unique_id"];
                $response["user"]["name"] = $user["name"];
                $response["user"]["email"] = $user["email"];
                $response["user"]["facebook_json"] = $user["facebook_json"];
                $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;
                $response["error_msg"] = "Unknown error occurred in registration!";
                echo json_encode($response);
            }
        }
    } else {
        $response["error"] = TRUE;
        $response["error_msg"] = "Required parameters (name, email or password) is missing!";
        echo json_encode($response);
    }

    ?>

Exact error: Uncaught Error: Call to undefined method mysqli_stmt::get_result() in register.php at this line: $user = $stmt->get_result()->fetch_assoc();