PHP Manual: Returns header for a message

string imap_fetchheader ( resource $imap_stream , int $msg_number [, int $options = 0 ] )

This function causes a fetch of the complete, unfiltered » RFC2822 format header of the specified message.

imap_stream

An IMAP stream returned by imap_open.

msg_number

The message number

options

The possible options are:

FT_UID - The msgno argument is a UID
FT_INTERNAL - The return string is in "internal" format, without any attempt to canonicalize to CRLF newlines
FT_PREFETCHTEXT - The RFC822.TEXT should be pre-fetched at the same time. This avoids an extra RTT on an IMAP connection if a full message text is desired (e.g. in a "save to local file" operation)

Returns the header of the specified message as a text string.

imap_fetchheader