PHP Manual: Unmark the message which is marked deleted

true imap_undelete(IMAP\Connection $imap, string $message_nums, int $flags = 0)

Removes the deletion flag for a specified message, which is set by imap_delete or imap_mail_move.

imap: An IMAP\Connection instance.
message_nums: A string representing one or more messages in IMAP4-style sequence format ("n", "n:m", or combination of these delimited by commas).
flags

Always returns true.

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

imap_undelete