what you can do is configure your apache to parse .html files so these files get parsed for php then make an include file to include at the top of each of these pages. this file woudl contain thefollowing:
note: there's a special variable containing the referring webpage.. if memory serves, it's $HTTP_REFERER, but I'm not sure.
in pseudo code:
if ($HTTP_REFERER is from this webserver) {
show them the page in this file(do nothing)
} else {
header("Location: http://dfd.df.ff"); // redirect them iftheyre from another site such as a search engine
}