IMAP Problem



i'd like to know which message i had not been seen, so i searched the manual and fid 2 flags says Unseen == 'U' || Recent == 'N' mean.
but i don,t understand what does Unseen == 'U' || Recent == 'N' mean(what he mean by flags)?
i want to check if the message is not seen, so how to do that?
i tried
$inbox = @imap_open("{}", "".$_SESSION['SESSION_USER_NAME']."+".$_SESSION['SESSION_MAIL_HOST']."", $_SESSION['SESSION_USER_PASS']) or header("Location: error.php?ec=3");
$headers = imap_header($inbox, $x);
if($headers->Unseen == 'U' || $headers->Recent == 'N')
but always show the message as unseen!!!!
i did gsef but i did not understand.they did not say how to use Unseen == 'U' || Recent == 'N' mean.

1- let me ask should i set the seen flag on the viewmail page?!!!!!
2- the function imap_setflag_full:
bool imap_setflag_full ( resource stream, string sequence, string flag, string options)
i saw sometimes he wrote the sequence as "2,4:6" and sometimes as "2,5"?!!!!
so how to write the sequence?!