If you insist on using imap_fetchbody() to retrieve a mail body that doesn't contain 'parts' [normally you'd use imap_body() ], then note that the header text is in part '0' and the body text in part '1'.
So, imap_fetchbody($mbox,$msg,'0') will return the header and imap_fetchbody($mbox,$msg,'1') will return the body text.