Not bad!
The design is simple, and straightforward. The colors aren't the worst possible either. The xhtml is valid, and the layout is obvious.
There are a few css validation errors, but they look simple to fix:
http://jigsaw.w3.org/css-validator/vali ... umber.net/
The redirect that you use to send people to the link image uses a HTTP 302 response. Arguably, I think the standard would prefer a 303, but a 302 isn't "wrong", since many http-1.0 clients don't understand 303. (
http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html - scroll down to 303).
I would change the wording when you add a new link from ("Data entered successfully. Your number is XX.") to ("Link entered successfully. Your link number is XX.")
I'd offer a GET link to users as well - having to type in a link number on a form isn't ideal.. I'd want to be able to do like..
http://linknumber.net/?page=25 or some such.
The error codes it returns are a little unclear. If the linknumber doesn't exist, it doesn't tell you - it just returns a blank page. Worse, if it is defined, but isn't working right, you get a standard apache error message. Those could be improved on.
It even does well in the colorblind web page test.
All in all, very solid. Well done.