An IMAP stream returned by imap_open().
ref should normally be just the server specification as described in imap_open()
Specifies where in the mailbox hierarchy to start searching.
There are two special characters you can pass as part of the pattern: '*' and '%'. '*' means to return all mailboxes. If you pass pattern as '*', you will get a list of the entire mailbox hierarchy. '%' means to return the current level only. '%' as the pattern parameter will return only the top level mailboxes; '~/mail/%' on UW_IMAPD will return every mailbox in the ~/mail directory, but none in subfolders of that directory.