[Brando]

It depends on which PHP bulletin board system you are using. vBulletin (this forum) has a PHP version, there's phpBB, Invision Power Board, PHPNuke has one built in. Some have the ability for you to load "plugins" - one of which would be CAPTCHA or CAPTCHA-like integration. Basically it sets a session cookie and an image output. The really good ones detect by region or user-agent to prevent a bot from just reading the cookie data and submitting that.

If you tell us what bulletin board software you are using, we may be able to guide you to a solution.