Stefan Mischook shares a little question and answer he had recently concerning object properties in PHP.
The question asked about accessing properties outside the class and/or defining them as protected/private to prevent it. Stefan's response was basically:
Many OO techniques are designed for situations where you will have more than one programmer involved, now or later. By declaring variables as protected or private, you are adding security to the code base by forcing the use of getter and setter methods where you can control how objects are used.