Usually when you see something like this, it means that you are including another file based on what $id is equal to.
Let's say you have 3 files: index.php, hello.php & goodbye.php:
index.php says:
if (file_exists("$id.php")) {
include "$id.php";
}
else {
print "Page Not Found";
}
Then if you go to http://www.whatever.com/index.php?id=hello, it will include whatever is in hello.php into index.php. And if you go to http://www.whatever.com/index.php?id=goodbye, it will include whatever is in goodbye.php and so on...
Hope that helped!