The object you get from imap_rfc822_parse_headers differs from the object you get from imap_headerinfo/imap_header in the following points. You won't get the:
* flags
* msgno
* size
* Maildate
* udate
* fetchfrom
* fetchsubject
udate can be simulated with
$headerobj=imap_rfc822_parse_headers ($header);
$udate=strtotime($headerobj->date);