imap_fetchbody - size limit?

Is there any limitation on the size of attachment that IMAP (especially functions like imap_fetchbody) can handle? I am writing a PHP script that will cycle through a number of email accounts and downloads new messages, storing the subject/body etc in a database and saving the attachments to the (local) hard disk. The script works fine with a 3MB attachment, but fails for a 4MB one - the file that it saves for the larger file has zero length (the script also work fine for a message with multiple 3MB attachments). As I understand it, there is no limit to the size of a variable's contents within PHP, so I am assuming that this is an issue with IMAP. The script is running on XP, and is being run on the command line. Thanks Andy Clarke