When imap_search() string contains no whitespace (ex: 

imap_search($mbox, "SUBJECT \"1234\"");

), it doesn't include the double-quotes in the command to the imap server.

The search above would result in this command to the imap server (verified with a packetsniffer):

SEARCH ALL SUBJECT 1234

If you run UW's imapd, this is not a problem, however, courier-imap requires double-quotes around the searchstring. Some strings work, but strings containing special characters or only numbers will fail if they are not surrounded by double-quotes.

If you instead do the following search:

imap_search($mbox, "SUBJECT \"1234 1234\"");

it will result in:

SEARCH ALL SUBJECT "1234 1234"

/pb