It is really a no-brainer to want to take some common CSS elements and simplify the process of changing them or updating them. Before we get any further though, let me stress that this is a limited use technique. The very nature of CSS is to cascade, which would be defeated by this if you went nuts with it.
So lets take a very simple example; we want to define the base color scheme and font families we want to use for body, h1 and our menu. Lets move those into PHP so that we can update them all with one small change.