Using PHP with Style Sheets

I just learned that you can even use PHP on style sheets. Can anyone here tell me how to set it up? I found a page at http://www.webdeveloper.co.nz/forum/topic/50 that apparently cites two different methods. The first is to put a php heading at the top of a style sheet, then alter a document so that your server will recognize the style sheet as a php page. But someone else said all you have to do is change the page's extension from .css to .php. I tried that, but it didn't work for me. Are you aware of any good tutorials on using PHP with style sheets? Thanks.