PHP in CSS?

They're independent technologies. Why would you want to use PHP in CSS?

The best I can think of is for some kind of conditional formatting, for which you may use PHP with CSS just as you would use PHP with HTML. PHP effectively doesn't know the difference between HTML and CSS -- it's all just text.

I have seen people do the following:

<link rel="stylesheet" type="text/css" href="style.php" />'

I haven't tried this out for myself though. Another way of doing this would be to have the CSS in the tags.

Or you could have PHP print the path to your chosen stylesheet:

I am thinking of conditional formatting, and maybe some stuff where I have to have ten buttons, all the same code other than background color. I could do some type of for loop.

Also, do you think it would still validate?

Yeah, it would validate. This is how server-side theme switchers, "currently selected" nav elements and such are done.

PHP doesn't know what it's printing out, as it's only text as far as it's concerned. The validator doesn't know about any PHP or server-side code, as it's only the output it can see.

You could use PHP to generate the buttons with classes assigned as you would normally do with HTML...you would not need to do anything differant with your CSS.
My site for example the main menu and the footer are generated by the same PHP script....the only problem I have at the moment with it is that it dose not validate because I am using an id tag for the active link and am having a little trouble getting it to work with a class tag....if anyone has any ideas.