IMAP, PHP不返回任何消息

我用我的用户名和密码连接到服务器。它连接的资源id为2。它可以正确显示状态，列出收件箱中未读电子邮件的数量，最近的邮件等。但是当涉及到imap_num_msg()函数时，我传递了正确的参数，但它返回0。我做错了什么让它返回0?

 <?php        
  $mbox = imap_open("{imap.gmail.com:993/imap/ssl}INBOX", "xxxxx", "xxxxxx",   OP_HALFOPEN)
or die("can't connect: " . imap_last_error());
   $list = imap_list($mbox, "{imap.gmail.com:993/imap/ssl}INBOX", "*");
   if (is_array($list)) {
   foreach ($list as $val) {
    echo imap_utf7_decode($val) . "'n";
   }
} else {
echo "imap_list failed: " . imap_last_error() . "'n";
}
 $status = imap_status($mbox, "{imap.gmail.com:993/imap/ssl}INBOX", SA_ALL);
 if ($status) {
   echo "Messages:   " . $status->messages    . "<br />'n";
   echo "Recent:     " . $status->recent      . "<br />'n";
   echo "Unseen:     " . $status->unseen      . "<br />'n";
   echo "UIDnext:    " . $status->uidnext     . "<br />'n";
   echo "UIDvalidity:" . $status->uidvalidity . "<br />'n";
  } else {
    echo "imap_status failed: " . imap_last_error() . "'n";
  }
  $numMessages = imap_num_msg($mbox);
 if ($numMessages) { 
   echo $numMessages;
 } else {
    echo "imap_status failed: " . imap_last_error() . "'n";
 }
imap_close($mbox);
?>