Discuz! X2验证码的产生及验证

http://www.mcqyy.com/wenku/jiaocheng/jzjc/cjc/106729.html

http://blog.sina.com.cn/s/blog_4acbd39c0100x571.html

http://www.discuz.net/forum.php?mod=viewthread&tid=2639646

http://www.discuz.net/thread-1692749-1-1.html

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http://www.discuz.net/thread-1670834-1-1.html