The following is valid for PHP 4 and later only.
Sometimes it is useful to refer to functions and variables in base classes or to refer to functions in classes that have not yet any instances. The :: operator is being used for this.
The above example calls the function example() in class A, but there is no object of class A, so that we cannot write $a->example() or similar. Instead we call example() as a 'class function', that is, as a function of the class itself, not any object of that class.
There are class functions, but there are no class variables. In fact, there is no object at all at the time of the call. Thus, a class function may not use any object variables (but it can use local and global variables), and it may not use $this at all.
In the above example, class B redefines the function example(). The original definition in class A is shadowed and no longer available, unless you are referring specifically to the implementation of example() in class A using the ::-operator. Write A::example() to do this (in fact, you should be writing parent::example(), as shown in the next section).
In this context, there is a current object and it may have object variables. Thus, when used from WITHIN an object function, you may use $this and object variables.