I want to use PHP includes for my site, but I'm confused. I have a CSS code, but do I have to make that into a PHP code to use it for PHP includes?

When I did my PHP includes, all I did was change the extension of all my pages to .php. Then I copied everything that goes above my content area into a page I called "header.php" and everything below into "footer.php." And finally I put the little include codes in place of those areas on all my pages. I could be completely wrong, but I don't think you have to change any code.

^Yep that's how you do it.

Read this. It's really simple, more simple than you think.

Go ahead
http://www.christopherjason.com/articles/using-php-includes/