To see the content of the returned object try this code:
$stream = imap_open($server, $username, $password);
$header = imap_fetchheader($stream, $uid, FT_UID);
$obj = imap_rfc822_parse_headers( $header);
print "<html><head></head><body><pre>\n";
print_r($obj);
print "</pre></body></html>";
imap_close($stream);