Blind shot - this is impossible because headers get sent as soon I they are encountered and thus you can override them by sending one more yet cannot remove what you have already sent. I might be wrong, though.

If I'm not then lots close this one. 

If you can <i>modify</i> headers using the header() function, they can't be sent already, can they?

At least with some types of headers, this works.

So I assume that headers are buffered at least until the document output has begun.

in php manual:
> The optional replace parameter indicates
> whether the header should replace a previous similar
> header,
> or add a second header of the same type.
> By default it will replace, but if you
> pass in FALSE as the second argument you can force
> multiple headers of the same type

so it can "replace" header, is should be able to "unset" header

header() is to "low level"
better to implement these functions:
header_set($name, $value [, response_code]) // replace or set new one
header_add($name, $value) // "append" new one
header_unset($name, [, $count]) // remove all, or N

these functions will work, until header_sent(), after which will get warnning.

it's just like operating on an array/table (but allow dup key)

by the way
php document never said: "headers get sent as soon they are encountered"

and it seems different for each sapi module

it's better for php to buffer all response headers until give it to sapi
and use:
header_flush();
to flush headers, but keep header_sent() false, which meaning header is not ended yet
header_flush is only used to "ping" client to see if connection lost, before output any content.

u may say, i can buffer it in script using array, but this is not able to manage php's internal headers

Please try using this CVS snapshot:

  http://snaps.php.net/php5.3-latest.tar.gz
 
For Windows:

  http://windows.php.net/snapshots/

http://news.php.net/php.cvs/54228