Hi Raven,
I don't fully understand what you mean, but my only guess is that if the ID is not defined, then you want to automatically include the "content.html"; really I need to see the rest of your code to see exactly how you get the page from the ID, or if you're trying to grab someone else's website from the ID...
You would need to use an if statement to validate that the ID is not null, the easiest way to do this is probably...
$id = $_GET['id'];
if (!isset($id))
{
// The ID is not set, therefore we would then load the "Content.html" page...
} else {
// The ID IS set, therefore we continue with the script as normal...
}