如何将单个列转换为JSON数组

Question

我正在编写一个Android应用程序，当用户登录时，我还想发送一个MySQL数据库列作为JSON响应的一部分。如何将MySQL列信息转换为数组，如何将其包含在JSON响应中？这是我的尝试：

以下是我的PHP代码：

if ($tag == 'login') {
        // Request type is check Login
        $email = $_POST['email'];
        $password = $_POST['password'];

        // check for user
        $user = $db->getUserByEmailAndPassword($email, $password);
        $groups = $db->getUserGroups();
        if ($user != false) {
            // user found
            $response["groups"] = $groups;
            $response["error"] = FALSE;
            $response["uid"] = $user["unique_id"];
            $response["user"]["name"] = $user["name"];
            $response["user"]["email"] = $user["email"];
            $response["user"]["user_group"] = $user["user_group"];
            $response["user"]["created_at"] = $user["created_at"];
            $response["user"]["updated_at"] = $user["updated_at"];
            echo json_encode($response);

/**
     * Get user by email and password
     */
    public function getUserByEmailAndPassword($email, $password) {
        $result = mysql_query("SELECT * FROM users WHERE email = '$email'") or die(mysql_error());
        // check for result 
        $no_of_rows = mysql_num_rows($result);
        if ($no_of_rows > 0) {
            $result = mysql_fetch_array($result);
            $salt = $result['salt'];
            $encrypted_password = $result['encrypted_password'];
            $hash = $this->checkhashSSHA($salt, $password);
            // check for password equality
            if ($encrypted_password == $hash) {
                // user authentication details are correct
                return $result;
            }
        } else {
            // user not found
            return false;
        }
    }
    /**
     *Return the user_group column of mysql database
     */
    public function getUserGroups() {
        $groups = mysql_query("SELECT user_group FROM users") or die(mysql_error());
        return $groups;
    }