this does look a bit confused ...
first, if your variable is $side, you should use $side instead of $page in the link I think.
then, if you use a fairly recent php version, you may need to use $_GET['$side'] instead of $side in your script.
then, the if-else constructs may be a good idea to make it look nicer, and the $sjekk variable is not really necessary.
<?php
if (isset($_GET['side'])) {
if (@fopen("include/".$_GET['side'].".php", "r")) {
include ("include/".$_GET['side'].".php");
} else {
include("404.php"); }
} else {
include("include/hoved.php");
}
?>