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
href="webcss.css" 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.