从PHP访问gmail垃圾邮件文件夹

如何使用PHP从Gmail帐户访问垃圾邮件文件夹?这里是我的代码连接到gmail的imap服务器。当我用SPAM替换INBOX时，会导致一个异常:

$mailbox = imap_open("{imap.googlemail.com:993/ssl}INBOX", "USERNAME@googlemail.com", "PASSWORD");
$mail = imap_search($mailbox, "ALL");
$mail_headers = imap_headerinfo($mailbox, $mail[0]);
$subject = $mail_headers->subject;
$from = $mail_headers->fromaddress;
imap_close($mailbox);