PHP - Which Is Safer?

I was using Include() with an ereg expression to strip all harmful characters out of the $_GET['id'];. However, a few people have stated to me that no matter what I do this function is not safe to use and is easily exploitable even after removing illegal characters from $_GET['id'];

Now I am looking for a way to include a web page that uses html and php. file() does not work as it includes my php code within the output so anyone can read it. File_get_contents() did not display any of the page whatsoever.