mb_encode_mimeheader function can't deal with long non-English strings?

Hi all, I wonder why mb_encode_mimeheader function can't deal with long non-english strings. Here are the codes: > function util_encode_mimeheader($headername,$str,$charset) { > if (function_exists('mb_internal_encoding') && > function_exists('mb_encode_mimeheader')) { > $x = mb_internal_encoding(); > mb_internal_encoding("UTF-8"); > $y = mb_encode_mimeheader($headername . ": " . $str, > $charset, "Q"); > mb_internal_encoding($x); > return $y; > } > } According to my tests, if the subject have too many characters, there will be more than one "?UTF-8?Q??" generated which make the MIME header invaild. If there are more than one "?UTF-8?Q??" , the remaing "?UTF-8?Q?"s will appear in the body of email, i.e. the beginning of the email will be: > " =?UTF-8?Q?=E6=9C=89=E9=97=AE=E9=A2=98?= > Content-type: text/plain; charset=UTF-8 ....". Can anyone explain this? Thanks!