How to achieve the creation of immutable structures?

Question

PHP

Good afternoon,
I can not understand how I can do this in this example ..

$a = RUB(10)->mul(5); 
$b = ($a->add(USD(5))->sub(RUB(3)))->mul(2);
$b->describe(); // возвращает строку '((10RUB) * 5 + 5USD - 3RUB) * 2'
$a->describe(); //(10RUB) * 5

I did but I change the object in the course of work,
as a result in $a->describe(); I got a value higher than and i.e. '((10RUB) * 5 + 5USD - 3RUB) * 2'
but //(10RUB) * 5
Thank you all for your help!

Answer 1

Oh somehow not correctly at you the immutable object is implemented. Read how to organize them in PHP, for example in this article