OOP PHP - 简单会计/财务系统的潜在类

I am kind of new to OOP in Php (but possess a good understanding of oop in java) and want to build a simple accounting system that will keep track of the business expense and revenues (generated from selling some service) and produce a monthly balance sheet(revenue-expense).

The mysql database is quite simple with the following tables:

user - login_id(pk), name, pass etc
customer - id(pk), name, tel etc
income - bill_no(pk), cust_id, description, date, amount etc
expense - receipt_no(pk), description, date, amount

I have already made significant progress with the following files (i am just showing the basic structure)

config.php
----------
    class DB_Class {
        function __construct() {
          //connect to db or die
        }
    }

functions.php
-------------
    class User{
        public function __construct() {
            new DB_Class();
        }
        public function register_user(params){
        }
        public function check_login(params){
        }
        public function get_session(){
        }
        public function user_logout(){
        } 
    }

registration.php
----------------
//html form
//new User() -> register_user(params)

login.php
---------
//new User() ->get_session()
//new User() ->check_login()

index.php
---------
//check session

My question is, what can be the other basic/must have/potential files, classes and their methods for such kind of system? I understand that it varies widely according to the system design, but any feedback/suggestion will be a very valuable input for me at this stage.

functions.php - strange name for a user class file, call it user.php. If you're holding several classes in a single file, separate them.

Possible extensions: you might want to make Expense and Income abstract and then specialize concrete incomes and expenses.

You might also want to have a class to model a Service which is sold.

Attention - possible clash if you want to give system access to customers - than you have overlapping between Customer and User class.

These are just some hints, hard to tell something more without more info about the domain rules, system restrictions, design, etc.