"Bloggers hate automated comment spammers. One way to foil these
vermin is a system called CAPTCHA, which is described in Wikipedia
as 'an acronym for 'completely automated public Turing test to tell
computers and humans apart''--in other words, a challenge-response
system designed to determine whether a site visitor is human or a
bot. Here's how I implemented a CAPTCHA system for blog comments
using free and open source (FOSS) command-line utilities and PHP,
the Web server scripting language.
"I've implemented this system under two Linux
distributions--Ubuntu Dapper Drake and CentOS 4--both running the
current versions of the Apache Web server and PHP. The CLI
utilities are from GNU Enscript and ImageMagick..."
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