Like I said earlier, it doesn't sound like PHP was installed with IMAP support enabled.
Try creating a new file, and put this in it:
phpinfo();
Then load the file and check to see if the IMAP module has been loaded...
If your spelling of the function name is correct, that's what you need to start looking...
P.S. My phpinfo() shows "mod_imap" under the loaded modules section of the ApacheHandler section... (And my IMAP functions work fine), but I am on a WinBox too...