ERP class diagram to solve real life OOP case

ERP_RealLifeCaseSolve.php

<?php

class Erp
{
    public function init($login)
    {
        //Which module
    }
    public function create($data)
    {

    }

    public function read($id)
    {

    }

    public function update($id, $data)
    {

    }

    public function delete($id)
    {

    }
}

class Module
{
    //Will apply some rules to load any specific module. Turn off, turn on any specific module to load
    public function load($module)
    {

    }
}

class Sales
{
    //Basic sales configuration will be here like which sub modules will be loaded. Sales managers info, etc...
}

class Customer extends Sales
{
    //All customer related operation means read, write data from ERP class
}

class Billing extends Sales
{
    //All billing operation means read, write data from ERP class
}



/**
 * ERP is the main class that can perform CRUD after init it
 *
 * Module => ERP has many modules. We will have to choose one module at a time. i.e: Sales, HR, etc...
 *
 * Sales module has more 2 sub-module like Customer, Billing, etcc....
 *
 *
 *
 * Now suppose I want to do a read operation on Customer Submodule like this way
 * 
 * $data = Erp->init()->Sales()->Customer()->read(1);
 * 
 * or what can be a the best practice?
 *
 */