This function causes a store to delete the specified
flag to the flags set for the
messages in the specified sequence.
The flags which you can unset are "\\Seen", "\\Answered",
"\\Flagged", "\\Deleted", and "\\Draft" (as defined
by RFC2060). 如果成功则返回
options are a bit mask and may contain the single option:
ST_UID- The sequence argument contains UIDs instead of sequence numbers
See also: imap_setflag_full().