Code to confirm adult visitors

I am looking for "that" nice code you see on some web sites. Basically it is the same as the javascript alert only fancier. Here is how it works: when a visitor enters the site the page is blocked with a transparent cover and there is a popup to confirm you are of the right age (or other purposes). Here is an example, click on the "Member Login" to see how it works: [login to view URL] I would need such thing and willing to pay. The code needs to work flawlessly with any php scripts and ideally with .html files too. A plus would be if it could work based on IP so it does not show again on the same IP.