check max two checkboxes while maintaining unique $_SESSION PHP

I have this working code, but now I have to make sure the user can only check a maximum of two checkboxes.

if (isset($_POST['submit'])) {
if (isset($_POST['steady_state']) ||
    (isset($_POST['ssrates']) ||
        (isset($_POST['unsteady_state']) ||
            (isset($_POST['usgas']) ||
                (isset($_POST['centrifuge_single']) ||
                    (isset($_POST['centrifuge_multi']) ||
                        (isset($_POST['porous'])))))))) {
    header("Location: /portal.php");

    $_SESSION['steady_state_portal'] = $_POST['steady_state'];
    $_SESSION['ssrates_portal'] = $_POST['ssrates'];
    $_SESSION['unsteady_state_portal'] = $_POST['unsteady_state'];
    $_SESSION['usgas_portal'] = $_POST['usgas'];
    $_SESSION['centrifuge_single_portal'] = $_POST['centrifuge_single'];
    $_SESSION['centrifuge_multi_portal'] = $_POST['centrifuge_multi'];
    $_SESSION['porous_portal'] = $_POST['porous'];

So, when for example the first and third checkbox is checked, the first and third $_SESSION is set.

Now, for the maximum of checkboxes I tried to change all names of the checkboxes to: name="checkbox" and then set the $_SESSION to:

$_SESSION['steady_state_portal'] = $_POST['checkbox'] && value==='steady_state';

But when I check one checkbox, all the $_SESSION are set instead of the one applied to the value. I didn't work on the count yet, because first I have to set the $_SESSION right. Can this be done with the existing code or do I need JavaScript? In both cases I need some help in the right direction.