An IMAP stream returned by imap_open().
The path to the saved file as a string, or a valid file descriptor returned by fopen().
The message number
The part number. It is a string of integers delimited by period which index into a body part list as per the IMAP4 specification
A bitmask with one or more of the following:
FT_UID- The msg_number is a UID
FT_PEEK- Do not set the \Seen flag if not already set
FT_INTERNAL- The return string is in internal format, will not canonicalize to CRLF.