I'm new to OOP/PHP. What's the practicality of visibility and extensibility in classes?

Question

I'm obviously brand new to these concepts. I just don't understand why you would limit access to properties or methods. It seems that you would just write the code according to intended results. Why would you create a private method instead of simply not calling that method? Is it for iterative object creation (if I'm stating that correctly), a multiple developer situation (don't mess up other people's work), or just so you don't mess up your own work accidentally?

Answer 1

Your last two points are quite accurate - you don't need multiple developers to have your stuff messed with. If you work on a project long enough, you'll realize you've forgotten much of what you did at the beginning.

One of the most important reasons for hiding something is so that you can safely change it later. If a field is public, and several months later you want to change it so that every time the field changes, something else happens, you're in trouble. Because it was public, there's no way to know or remember how many other places accessed that field directly. If it's private, you have a guarantee that it isn't being touched outside of this class. You likely have a public method wrapped around it, and you can easily change the behavior of that method.

In general, more you things make public, the more you have to worry about compatibility with other code.