Well first off... comparing CSS to PHP is like apples to oranges... so lets just think for a moment and decide to compare <font> tags vs. CSS instead, cause that makes more sense.
CSS is FAR LESS LABORIOUS than individual <font> tags
With CSS, people can specify their own stylesheets
With CSS, you can have multiple stylesheets for diff. browsers
With CSS, you can change the entire look of a site by just changing the stylesheet, no need to edit code at all.
And the big and important one...
<font> tags have been deprecated in the X/HTML spec and ( not sure on this ) possibly in HTML 4.0 too, so DON'T USE THEM, YOU'RE NOT SUPPOSED TO!
Future browser versions more than likely won't support <font> tags AT ALL, and will require CSS.