I understand that you want to avoid spam in your forum by adding this code. I would recommend bbantipspam or Advanced Textual Confirmation, which adds an extra page upon posting, where you have to answer a question. Spambots cant pass this test and it also occurs only upon the first post.
bbantispam is easy to install and if the installation code is added in config.php it protects against all types of spam in PHP-Nuke and even in News comments.
Bbantispam works for both anonymous and registered users, but it is only working the first posting. If you pass the test it will not appear again in 2-3 weeks at least. bbantispam is considered safer than security code, because spambots can often read security code and pass it.
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