===cut===
<?php

 function open ($save_path, $session_name) {
    $f = fopen('log', 'a');
    fputs($f, "open ($save_path, $session_name)\n");
    fclose($f);
     return true;
 }

 function close() {
    $f = fopen('log', 'a');
    fputs($f, "close\n");
    fclose($f);
     return true;
 }

 function read ($key) {
    $f = fopen('log', 'a');
    fputs($f, "read ($key)\n");
    fclose($f);
     return "foo|i:1;";
 }

 function write ($key, $val) {
    $ip = getenv("REMOTE_ADDR");
    $f = fopen('log', 'a');
    fputs($f, "write ($key, $val)\n");
    fclose($f);
     return true;
 }

 function destroy ($key) {
    $f = fopen('log', 'a');
    fputs($f, "destroy($key)\n");
    fclose($f);
     return true;
 }

 function gc ($maxlifetime) {
     return true;
 }

 session_set_save_handler ("open", "close", "read", "write", "destroy", "gc");

 session_start();

 $foo++;

 ?>
===cut===

this will produce only one "log" only with "open" and "read", no "write" and "close"...

this is not only 4.0.5 bug, but 4.0.4pl1 too.