corrected/working patch is here now (previuos had errors ..I should test patches before submitting them :) ) 

http://www.flyspray.org/patches/setcookie-4096btyesonly.patch

http://www.faqs.org/rfcs/rfc2109
http://www.faqs.org/rfcs/rfc2965

RFCS linked from the setcookie function docs

"
...
     *  at least 300 cookies

      *  at least 4096 bytes per cookie (as measured by the characters
         that comprise the cookie non-terminal in the syntax description
         of the Set-Cookie2 header, and as received in the Set-Cookie2
         header)
...
"

keyword:at least 

so basicly php should not limit cookie length, it up to the client/browser how to handle it.

PHP implements the netscape spec, **not** the RFC one

"When a cookie larger than 4 kilobytes is encountered the cookie should be trimmed to fit, but the name should remain intact as long as it is less than 4 kilobytes"

"Servers should not expect clients to be able to exceed these limits"


In the case of PHP, sending a cookie bigger than 4kb is useless because no browser will use it correctly, and truncating it without emitting any warning just makes debugging  harder.

also imagine the following code

setcookie ("foo", $_GET['reallybigdata']) 

it can also exausts the PHP process or system memory(dependding on the memory limit)for no gain because the browser will truncate it anyway.

ie7 increased the number of cookies per domain from 20 to 50, like other browsers.

so why not limiting php to set max 20 cookies per domain like NS "mention"?

Netscape also follow these rfc

if the client ignore part of the cookie it's not PHP concern.
what if I built an application follow the rfc and php artificially limiting me?

In my opinion it's up to the coder to decide what he allows to be put in cookies set. If he/she does't bother doing any filtering or such for and inserts blindly input data there, it's his/her fault..

While it would be absolutely trivial to add an artificial limit of 4K or 
any other arbitrary # as the maximum cookie length. This would not be 
correct and has a good chance of breaking existing applications.