I've used session_register("sErr") to register session variable. then it's used in following function:

function kBan_error() {
    global $HTTP_SESSION_VARS;
    if($HTTP_SESSION_VARS['sErr']):
	echo "<span class=\"error\">".$HTTP_SESSION_VARS['sErr']."</span>";
    endif;
    $HTTP_SESSION_VARS['sErr'] = "";
}

on another site the $HTTP_SESSION_VARS['sErr'] is empty. after calling session_destroy() is the variable empty again. but after calling session_start() in the same browser window, the variable sets to latest value that it's been filled with. calling the function kBan_error() twice in a row with defined variable $HTTP_SESSION_VARS['sErr'] causes only one appearance of the error message.