不同浏览器session_id相同的可能性？？？？
session_id重复 session cookie
我一直知道的是session_id绝对唯一。但在做一个二级域名的session共享的时候，发现一个问题：用户第一次访问时，将session_id保存到cookie，cookie有效期设置为一天,然后用户退出。在30分钟后，服务器的session被销毁，而此时用户携带被保存在cookie的session_id访问。。在服务器端，为了二级域名共享session_id,做了一个判断：只要存在$_COOKIE['session_id']，就使用session_id()函数直接访问服务器的session。。
那么问题就出现了，此时用户传递的session_id，对应的session已经被销毁了。那么此时服务器是否可能存在一个session，它的session_id恰好是用户传递的$_COOKIE['session_id']呢？？？
注：服务器销毁session后，自然会忽略掉该session对应的session_id。那么也就可能服务器再次生成这个session_id分配给另外的session。
求大礼赐教，不胜感激！！！！！
回复讨论(解决方案)
session_id 是一个与时间相关的值，只要地球还在转，就不会出现重复
时间是递增的值，所以已经过去的时间不可能在将来出现。但是session_id不是字母+数字的字符串吗，从排列组合上来说也存在重复的可能性啊？？
session_id 是一个与时间相关的值，只要地球还在转，就不会出现重复
每微秒产生一个数，8字节大约可以使用100年，一百年后电脑还是电脑吗？
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