首先解释一下什么是Session，如果你不知道的话。当一个访问者来到你的网站的时候一个Session就开始了，当他离开的时候Session就结束了。本质是来说，cookie是和浏览器有关系，而Session变量就可以存一些资源变量在服务器上面。PHP4用文件存储Session变量，但理论上可以用数据库或共享内存来做这件事。 所有的页面都用PHP4的Session必须用Session_start()功能函数来告诉PHP4引擎来取有关的Session到内存中 。函数Session_start()可以在cookie域里或请求的参数中取得Session_id为了响应http请求。如果不能找到 SessionID就新建一个Session。 什么是Session变量？ Session变量是个有规律的全局变量，当一个Session变量被注册，用PHP4可以在所有的页面上得到Session 的值。用Session_register("variable_name")可以注册一个Session变量。在所有并发的的用Session就使用 Session_start()函数，变量的值将作为一个Session变量注册为Session。 我们能作什么？ 通常有很多的方法来管理Session和Session变量，我将给你个例子。说你将建一个商业站点，象我这样的， 你可能想保持已经被承认的用户当前的名字，或有多少的新消息用户已经得到。为了不在从数据库里读取，你 有两个方法可以做： 1.1.你可以用三个cookie 。authenticated_user - 当前的用户名称 。num_messages - 他得到的信息的数量 。expire_time - 何时重新读取信息数量 2.2.用sessions和新建三个session变量 第一个方法安全性不好，一些人可以得到cookie进入他人的领域。 用sessions用户仅得到一个cookie，安全的多。 缺点 session给了你自由，过度的用session会影响脚本语言的使用。虽然PHP4的session有些限制，如你不能存对象 在session里。 自己试一试吧！
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