Um ...not sure why you would want to. Since PHP is a server based code and CSS "styles"the HTML on a browser level...what are you tring to accomplish???
You cannot use PHP or Javascript inside a stylesheet, but you can use them to change properties and create new ones. You can even switch stylesheets with just one line of PHP code.
You can. You save your stylesheet with .php extension and you can code with <?php and ?> perfectly. Also maybe you can use a header('Content-type: text/css'); Regards.
You cant use PHP code in a CSS file. You must have a .PHP file extension in order for a PHP code to execute properly, However you can create a CSS file with a PHP file extension, with that you can add your PHP code.
If your framework is wordpress you can place the css in the functions.php file and call the function in header.php.