You can do it like that, just put this code inside the <head> tags on the HTML page:
<link REL="stylesheet" HREF="style.inc.php3" TYPE="text/css">
However I would not recommend it... :<
I did it and unfortunately Internet Explorer (under Win98, IE 5.0, IE 5.5) hanged... Not on all machines but on few - with some specific configuration - every time...
After including the CSS file on the HTML page it worked fine.
Maybe the problem is that I linked the PHP file but specified the TYPE of the file as: "text/css"...
Maybe specifing it as "application/x-httpd-php" would fixed the bug?
Try this out.. if you will getting problems with IE just include the CSS, dont link it...