Is there a reason why I can't use True and False conditions on the same page?

I have three different conditions used to display specific content on the page. However the second condition seems to invalidate the third one. I'm wondering how I can fix this. Whether I put false or true first the third condition seems to be ignored. I have tried many different combinations and it's always the same thing. Whatever condition I put first after <?php if(isset($_GET['viewinbox']) == 'inboxmessages' && $resultm == TRUE OR $resultm == FALSE) :?> the third condition is ignored. I wonder why.

        <?php if(isset($_GET['viewinbox']) == 'inboxmessages' && $resultm == TRUE OR $resultm == FALSE) :?>

//show main page

<?php elseif($_GET['view'] == 'inboxmessage'  && $resultm != TRUE) :?>

//show my inbox

      <?php elseif($_GET['view'] == 'inboxmessage'  && $resultm == TRUE) :?>//whatever condition is placed here gets ignored


//show something else

<?php endif;?>