function myfunc($input,$page="overnatning.php") {
	$age1 = 18;
        $age2 = 30;
        $age3 = 45;
        $age4 = 60;
       	$side = 1;
        if($input >= $age1 && $input < $age2)
        {
        	$side=1;
        }
        if($input >= $age2 && $input < $age3)
        {
               	$side=2;
        }
        if($input >= $age3 && $input < $age4)
        {
                $side=3;
        }
        if($input >= $age4)
        {
                $side=4;
        }
	header("Location: ".$page."?side=".$side);
}

myfunc($_POST['alder']);

myfunc($_POST['alder'],"otherpage.php");