登录脚本以显示名字

Question

我正在使用一个完美运行的登录脚本，但我很难理解如何将first_name字段从我的用户数据库设置为一个会话并可以显示，据我所知它使用以下函数来制作登录：

function login_check($mysqli) {
    // Check if all session variables are set 
    if (isset($_SESSION['user_id'], $_SESSION['username'], $_SESSION['login_string'])) {
        $user_id = $_SESSION['user_id'];
        $login_string = $_SESSION['login_string'];
        $username = $_SESSION['username'];

        // Get the user-agent string of the user.
        $user_browser = $_SERVER['HTTP_USER_AGENT'];

        if ($stmt = $mysqli->prepare("SELECT password  FROM members  WHERE id = ? LIMIT 1")) {
            // Bind "$user_id" to parameter. 
            $stmt->bind_param('i', $user_id);
            $stmt->execute();   // Execute the prepared query.
            $stmt->store_result();

            if ($stmt->num_rows == 1) {
                // If the user exists get variables from result.
                $stmt->bind_result($password);
                $stmt->fetch();
                $login_check = hash('sha512', $password . $user_browser);

                if ($login_check == $login_string) {

                    return true;
                } else {
                    // Not logged in 
                    return false;
                }
            } else {
                // Not logged in 
                return false;
            }
        } else {
            // Could not prepare statement
            header("Location: ../error.php?err=Database error: cannot prepare statement");
            exit();
        }
    } else {
        // Not logged in 
        return false;
    }
}

我最初的尝试是在$_SESSION['login_string'])) {内添加会话变量，但它不是，并返回一个空值。我可以返回用户名和user_id，但仅此而已？