In another entry in his "Confessions of a Converted PHP Developer" series Mal Curtis about a typical class development path PHP developers take and how it can be a limiting, linear progression from one step to another.

In converting to Ruby I realized how strict PHP is in its class hierarchy. This surprised me as I'd always viewed PHP as a very loose language, mainly due to its loose typing and lack of formal code structure guidelines. [...] I find most PHP developers learn starting by using inline PHP as a low learning curve entry point into dynamic web languages and then move on to fuller, more complex, applications.

He includes a few code samples comparing how Ruby, while still allowing the same kind of structure, also lets you redefine classes on the fly. His example shows redefining a method in a Ruby class using a method commonly called "monkey patching". It's good that he points out a downfall of the approach too:

I must note that Monkey Patching is often a quick fix solution that can create headaches for future developers (or for yourself, if your memory is like mine) for several reasons. [...] Use at your own risk!

There's several different articles all over the web talking about the "monkey patching" approach and if it's possible/useful in PHP.