As far as you might know, $_GET, $_POST, $_COOKIE, also as $_SESSION were introduced only in PHP 4.1.0. This is a minimum requirement for running miniBB 2. But anyway, if you have version below (I've implemented this in 4.0.6), there are 2 simple steps for doing that:
1) Edit setup_options.php and paste at the very top of it (before $DB='mysql'; ) the following code:
/* PHP 4.0.6 specific */
if(isset($HTTP_GET_VARS) and is_array($HTTP_GET_VARS) and sizeof($HTTP_GET_VARS)>0) foreach($HTTP_GET_VARS as $key=>$val) $GLOBALS['_GET'][$key]=$HTTP_GET_VARS[$key];
if(isset($HTTP_POST_VARS) and is_array($HTTP_POST_VARS) and sizeof($HTTP_POST_VARS)>0) foreach($HTTP_POST_VARS as $key=>$val) $GLOBALS['_POST'][$key]=$HTTP_POST_VARS[$key];
if(isset($HTTP_COOKIE_VARS) and is_array($HTTP_COOKIE_VARS) and sizeof($HTTP_COOKIE_VARS)>0) foreach($HTTP_COOKIE_VARS as $key=>$val) $GLOBALS['_COOKIE'][$key]=$HTTP_COOKIE_VARS[$key];
/* PHP 4.0.6 specific */
2) Edit bb_cookie.php and in getMyCookie() function, declare $_COOKIE variable as global:
function getMyCookie(){
global $_COOKIE;
...........
It seems, that's all... | 
