session_set_save_handler()中write使用的有关问题

session_set_save_handler()中write使用的问题
我把SESSION写入数据库，sessionID用URL传。

a.php:

PHP code

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/--><?php  require('session.php');  session_start();  $_SESSION['test'] = 1;  echo '<a href="b.php?' . session_name() . '=' . session_id() . '">查看</a>';

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b.php

PHP code

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/--><?php  require('session.php');  session_start();

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session.php:

PHP code

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/--><?php    ini_set('session.save_handler', 'user');    ini_set('session.use_cookies', 0);        session_set_save_handler('open', 'close', 'read', 'write', 'destroy', 'gc');        $link = null;    $expiry = 86400;        function open($save_path, $session_name)    {        //连接数据库    }        function close() {        return true;    }        function read($sessionId) {        global $link;                $sessionId = mysql_escape_string($sessionId);        $time = time();        $sql = "SELECT value FROM session WHERE session_id='{$sessionId}' AND expiry>{$time}";                $result = mysql_query($sql, $link);                if(@$row = mysql_fetch_row($result, $link)) {            return $row[0];        }else {            return '';        }    }        function write($sessionId, $sessionData) {        global $link, $expiry;                $sessionId = mysql_escape_string($sessionId);        $sessionData = mysql_escape_string($sessionData);        $time = time() + $expiry;                $sql = "INSERT INTO session(session_id, value, expiry) VALUES('{$sessionId}', '{$sessionData}', {$time});";        $result = mysql_query($sql, $link);                if($result) {            return true;        }                $sql = "UPDATE session SET value='{$sessionData}', expiry={$time} WHERE session_id='{$sessionId}'";        $result = mysql_query($sql, $link);                    if($result && mysql_affected_rows($link)) {            return true;        }else {            return false;        }    }        function destroy($sessionId) {        global $link;                $sessionId = mysql_escape_string($sessionId);                $sql = "DELETE FROM session WHERE session_id='{$sessionId}';";        $result = mysql_query($sql, $link);                if($result) {            return true;        }else {            return false;        }    }        function gc($maxLifeTime) {        return true;    }

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现在是这样：
运行a.php，SESSION确实保存到了数据库中：
+----------------------------+--------------------------------+------------+
| session_id | value | expiry |
+----------------------------+--------------------------------+------------+
| o7olki3vmch3k8bakhk71emco6 | isLogin|i:1;permission|s:0:""; | 1344495859 |
+----------------------------+--------------------------------+------------+

问题是：
当点击a.php的超链接去b.php时，传过去的sessionID所代表的SESSION值被清空了。
+----------------------------+-------+------------+
| session_id | value | expiry |
+----------------------------+-------+------------+
| o7olki3vmch3k8bakhk71emco6 | | 1344496029 |
+----------------------------+-------+------------+

后来我发现，原来是运行b.php页面时，执行了write()方法中的UPDATE：
UPDATE session SET value='', expiry=1344496029 WHERE session_id='o7olki3vmch3k8bakhk71emco6'
但是我在b.php页面中并没有更改SESSION的操作啊？这是怎么回事呢？

麻烦大家了！！！


------解决方案--------------------
php 并不知道你是否修改了 session 变量的值
所以他总是要将 session 重写回去的