Set an email as SEEN on IMAP server

I am trying to read mail from an Imap Server (Gmail). I would check if there are new mail (unseen) and check it as seen.I wrote this code but

imap_setflag_full

seems to not work. If I have a new mail the script is unable to put the SEEN flag and it echo me that there is always one unseen mail.

  $mbox=imap_open( "{imap.gmail.com:993/ssl/novalidate-cert}" , $this->username, $this->password);    if ($mbox)             {  echo "Connected\n<br><br>";             }  else { exit ("Can't connect: " . imap_last_error() ."\n");  echo "FAIL!\n";  };         if ($hdr = imap_check($mbox)) {          $msgCount = $hdr->Nmsgs;          echo "There are ".$msgCount." mail";        } else {          echo "Failed to get mail";        }        $result = imap_search($mbox, 'UNSEEN');        echo "<br>Result: ";        print_r($result);        if($result==false)            echo "No email";        else{            echo "you have mail";             echo("<br>now I set the Seen flag for this mail");            rsort($result);            $status = imap_setflag_full($mbox, "1", "\\Seen \\Flagged", ST_UID);              }        echo"<br><br>";        $result = imap_search($mbox, 'UNSEEN');        echo "<br>Result: ";        print_r($result);        if($result==false)            echo "no mail";        else{            echo "there are still";         }

Thank you so much.