PHP 如何判断当前用户已在别处登录，当前用户在别处
出处：http://bbs.lampbrother.net/read-htm-tid-121909-ds-1.html#tpc
主要思路：
1.登录时，将用户的SessionID记录下来
2.验证登录时，将记录的该用户SessionID与当前SessionID匹配
3.如果不相同，说明在别处登录
具体请看demo代码，内有注释。
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