DevShed has posted the fifth part of their series looking at the visibility keywords on PHP5's object oriented support today. They've already looked at private, public and protected properties in a class, now they look at the use of making methods private to restrict their use/extension.
Of course, when it comes to specifying how visible a certain class property or method will be, you know that PHP 5 permits you to work with three distinct levels of access, called "public," "protected," and "private" respectively. [...] As you may have noticed, however, I've not taught you how to define private methods yet, which is something that can definitely be very useful if you want to restrict the access to your classes from the outside more severely.
The tutorial shows the creation of a class with private properties and then expands it to include a private method. Then they call it from an object, an example of the error PHP kicks back is there too. He also includes the concept of a "getter" to call the private function from a public one.