(PHP 4, PHP 5, PHP 7)
imap_uid — This function returns the UID for the given message sequence number
$imap_stream
, int $msg_number
)This function returns the UID for the given message sequence number. An UID is a unique identifier that will not change over time while a message sequence number may change whenever the content of the mailbox changes.
This function is the inverse of imap_msgno().
The UID of the given message.
Note:
This function is not supported by POP3 mailboxes.
I would like to point out that, at least in the case of Gmail, the UIDs of emails are changed if they are moved from one folder to another. Even if moved back to the same folder the UIDs get incremented and thus changed.
I found this the hard way after relaying on the notion that UIDs do not change as I found almost everywhere in the docs.
Be aware that the uid is unique in a single mailbox. That means a message can have as much uids as mailboxes
clarification for Andreas' note about imap_expunge()...
imap_expunge() does indeed change the UID, but only *after* the IMAP stream has closed and then reconnects.
You can use imap_expunge() as many times as you want within one IMAP stream (no reconnects) and the UID stays the same while the Msgno will change with imap_delete() followed by imap_expunge().
The uid will change in case of unsing imap_expunge. Be shure that
all work ist done before using imap_expunge.
A little quirk of UIDs in a PUBLIC folder in MDaemon (and perhaps other servers?). The UIDs may be unsynchronised between users if the server's option to store flags on a per-user basis is turned on. That is, a message may have a UID of 5 for one user (meaning it was the 5th message to enter the box) but 3 for another user (who perhaps subscribed to the box later). If this is a problem, turn off the per-user option for the mailbox.
Please note that message_id in the header is NOT the UID - and its dangerous to use as many mail progs don't put a message id in