会话文件权限 - 跨帐户读/写

Question

我正在尝试跨子域分享Sessions，这也是他们自己的帐户。

www.example.com 
kb.example.com

我用来执行此操作的代码是

<?php

    ini_set("session.cookie_domain", ".example.com");

     // this was setting the session file to 0644 even though i was attempting 0777
    //ini_set('session.save_path', '0;777;' . session_save_path()); 

    session_start();

    // this modifies the session files permissions correctly
    $path = session_save_path() . '/sess_' . session_id();
    chmod($path, 0777);

    echo '<pre>';
    print_r($_SESSION);
    echo '</pre>';

    $_SESSION['test'] = 1;

    echo session_id();

?>

如果我在www.example.com上运行此代码，可以很好地阅读会话信息，如果我在kb.example.com上运行相同的代码，我将收到以下错误

PHP Warning:  chmod(): Operation not permitted in /home/kbexample/public_html/test.php on line 8

PHP Warning:  Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0

如何在这些域中读取/写入会话数据？为什么我不能读取文件，即使它是0777？