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More than 60 PHP Guestbook are provided on PHPKode.com
http://www.phpkode.com/projects/category/php-guestbooks/

Find the most Security one to allow visitors to comment on your
webiste now? Maybe I should recommend you this one, GBook -
http://www.phpkode.com/projects/item/gbook/, But it should be better
if you can find one by yourself!