Howto use Thunderbird hack mode ("/tb") with PHP imap function?

[Herc]

I'm writing an e-mail migration tool in PHP to move old MBOX files to Zimbra via IMAP. Of course I have a "well known" INTERNALDATE problem, cos PHP imap_append() function doesn't let me set it. Instead I wanted to connect in Thunderbird Hack Mode cos then Zimbra would use "Date:" header instead of INTERNALDATE, right?
Well, how can I do it?

imap_open("{myserver.com}INBOX", "username/tb", "password") dosen't work.
imap_open("{myserver.com/user=username}INBOX", "username/tb", "password") dosen't work.
imap_open("{myserver.com/tb}INBOX", "user", "password") error
imap_open.....

I was guessing on and on but no luck. What does really TB does with "/tb" appended to username? It surly doesn't send it directly as username... or am I wrong?

Please, help