Why doesn't PHP permit private const?
Solution 1:
As of PHP 7.1, there are real private constants.
private const PRIVATE_CONST = 0;
See the Class Constant Visibility RFC for more information.
Solution 2:
Use private static properties.
In that case you will have the same variable throughout all objects and if you want to extend its scope to nested, you can expose a getter method to get its value and restrict variables settings.