One thing I would repeat in the docs here is what information actually comes from the user. Many people think a Cookie, since it's written by PHP, was safe. But the fact is that it's stored on the user's computer, transferred by the user's browser, and thus very easy to manipulate.
So, it'd be handy to mention here again that:
CGI parameters in the URL, HTTP POST data and cookie variables are considered "user data" and thus need to be validated. Session data and SQL database contents only need to be validated if they came from untrustworthy sources (like the ones just mentioned).
Not new, but I would have expected this info under this headline, at least as a short recap plus linlk to the actual docs.