In the prior article, PHP 5 OOP: Interfaces Abstract Classes and the Adapter Pattern, I presented an interface for abstracting database access from the actual database. The article also presented code for accessing a database.
In this article you will learn to expand upon the functionality you built from the first article in order to show how to protect your data using visibility modifiers.
They start with a look at some of the "accessibility modifiers" that PHP5 has to offer - like private or public - in it's object model. They define them in the context of their database class as well, demonstrating how both variables and classes can be marked this way. After this, it's all about the code, looking first at preparing the SQL query, then validate the SQL before sending it off to the last step - executing the query and fetching the results (if needed).