Php/OOP How to avoid instances of the same class to access private properties/methods from other objects

Simply don't write code which accesses other entities' privates, period. The visibility modifiers are there to help you not shoot yourself in the foot too easily. They're not a lock and key. There are any number of ways in which you can still "circumvent" "access protection". Just be a responsible adult and not modify properties except when you write a $this-> before it.