Thought I should clarify my post somewhat. css content is probably not the way to go. I, personally would not use it as there are so many users without v8+ of IE. Grr, pesky Microsoft! I'd go along with pritaeas - a templating system is what you're looking for - this can be as simple as having include files (require/include) or as complicated as a full-blown templating system (smarty, rain tpl etc). Anyway, I just wanted to suggest - don't use the css option for a year or so, until IE8 uptake by IE users is close to saturation (might never happen!).
Just so you know, this is not true. On apache you can use any extension you want and treat it as a php file.
Just so you know, this is often functionally true. apache server is configurable, however, many servers, particularly low end hosting packs, are locked down
/* addhandler server-parsed is disabled */
so that configuration cannot be changed,
plan on the worst, and you get pleasant surprises when you find the result is better than you expect
plan on the best, and the only surprises are unpleasant