Maybe there are plenty of ways. You can assign value to $_GET key if one does not exist. Or if you really need to query string, you can renavigate the user to the same page with present querystring.
if (!isset($_GET['id'])) {
header("Location: Page.php?id=1");
exit;
}
It should be before any output in the page. So if user visits Page.php
or Page.php?
or Page.php?someDifferentParamThanId=10
it will return false on isset($_GET['id'])
thus it will redirect to Page.php?id=1