为了方便使用php的session，我在这里重写了一个简单的session方法。

新建application/libraries/Sessions.php，内容如下：

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<?php
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if (!defined('BASEPATH')) exit('No direct script access allowed');
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/**
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* Reconstruct the session class
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* @author chory
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* @version 1.0
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* @copyright 2011/6/12
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*/
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class Sessions{
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    private static $instances;
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    private static function instance()
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    {
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        if (empty(self::$instances)){
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            @self::$instances = &load_class('session');
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        }
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        return self::$instances;
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    }
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    public static function set($key, $value = "")
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    {
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        self::instance() -> set_userdata(array($key => $value));
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    }
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    public static function get($key = null)
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    {
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        if ($key)
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        {
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            return self::instance() -> userdata($key);
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        }
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        else
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        {
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            return self::instance() -> all_userdata();
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        }
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    }
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    public static function _unset($key){
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        self::instance() -> unset_userdata($key);
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    }
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    public static function destroy(){
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        self::instance() -> sess_destroy();
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    }
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}
应用方法：

Sessions::set("username", "admin");

Sessions::get("username");

可以在autoload.php中设置自动加载，或手动调用Sessions