PHP Manual: imap_fetchheader

function imap_fetchheader(IMAP\Connection $imap, int $message_num, int $flags = 0): string|false

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

imap

An IMAP\Connection instance.

message_num

The message number

flags

The possible flags are:

FT_UID - The message_num 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, or false on failure.

Version	Description
8.1.0	The `imap` parameter expects an IMAP\Connection instance now; previously, a valid `imap` resource was expected.

imap_fetchheader