This hasn't a thing to do with PHP.
Moving from PHP forum to HTML/CSS.
It doesn't matter of the CSS is in the same directory as the PHP file. Since you are linking it in using the <link> html tag, you must place it in the path governed by the html root since its relying entirely on the client to access this. So with the just the
that would indicate that webcss.css must be located directly off of the html root which is probably your ~/public_html directory.
Actually, without the / in front of the path, I'm not 100% sure what the client does to resolve this (absolute or relative from the html root). I'd expect still off of the html root, but the client guys can verify.