using inheritance to build class hierarchies,

As you can see, the controlling logic above looks for a $_GET['category'] variable.

If it exists, it creates an instance of CategoryHeader, displaying the navigation

to allow users to find their way back to the home page. But if it doesn't exist,

it creates an instance of the parent StandardHeader instead, which applies when

users view the home page (and therefore does not require bread crumbs to find

their way back).

In other words, inheritance allows us to add the extra functionality we need

without having to reproduce the logic that already resides within the parent class;

the existing methods and logic can be reused via the child subclass.

Inheritance provides a powerful mechanism to make classes that are modular,

addressing a specific problem, while still making available shared methods and

variables that can be used irrespective of the specific object we're dealing with.

Avoid Deep Inheritance Structures

As a general rule of thumb, when using inheritance to build class hierarchies,

avoid going deeper than two generations.

Inheritance in Action

Doing so is often a sign of a bad design, in which opportunities for classes

to interact in different ways (see the next solution) were missed. In practice,

having more than two generations of classes often leads to all sorts of debugging

problems and makes the code difficult to maintain. For example, it can

become hard to keep track of variable names you've used higher up in the

hierarchy.