Using IF statements based on userid and user logged in

I have a simple website with a profile page for users. When a user logs in, their page is displayed on profile.php a specific user page is access by profile.php?userid=2

I am trying to achieve a set up like this..

Users Logged In can access their profile by: profile.php

profile.php?userid=2

users that are logged out:

profile.php I want to be blank.

and profile.php?userid=2 to still be accessible.

I've managed to get a variation of this working and not working in places.

my code right now to view profiles.

if($_GET['userid']!='') {
$profileuserid=$_GET['userid'];
} else {
$profileuserid=$_SESSION['userid'];
}

To view profile.

<?php if($_SESSION['userid']==$profileuserid){ ?>
//This is where my content goes.
<?php } ?>

Then to try and achieve my goal I was using these.

 <?php if(($_GET['userid']==$profileuserid) && ($_SESSION['loggedin'])) { ?>

<?php } elseif((!$_GET['userid']==$profileuserid) && ($_SESSION['loggedin'])) { ?>

<?php } elseif(($_GET['userid']=$profileuserid) && (!$_SESSION['loggedin'])) { ?>

<?php } ?>

Thanks in advance.