That is a different question than what I read.
The answer, I believe, lies in what school of programming you come from. For instance in C++ the default access level of a property is private. In many circles private and public are nearly the only considered access levels of properties. Many schools of teaching in OOP teach in this manor. There are others that think public and protected should be where the sweet spot is.
PHP 5's choice was to make the default public which I suspect has more to do with legacy code from PHP 4 than in growing the language, but that's neither here nor there. This functions much more like C's structs when function tables are added.
It is my personal preference to use private more than protected and public. You are free to choose whichever preference you like. I certainly won't try to stop you.
Now, someone will likely ask why I do that. The reason I do it is partly due to how I was taught object oriented programming but also in my feeling that most classes should be considered black boxes, even from derivative code. This stems from using and building large application frameworks with hundreds and hundreds of classes which could be developed by any number of individuals over a long period of time. For me, it's a way to keep things consistent throughout the system.