Forcing human creation of emails

I need to put on our website the ability to force someone sending us an email to type in 4 or 5 random characters to confirm that it's human generated.

For some reason, I cannot remember what this technique is called.

All pages are in php, so a php version would be helpful.
Richard KortsBusiness Owner / Chief DeveloperAsked:
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Depending on the CMS you are using you might try a CAPTCHA module like Mollom for Drupal, a php based CMS.

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I know this was answered already and points accepted, but I wanted to second Mollom. It was created by the creator of Drupal, and it's not obtrusive. That is, it doesn't even present a CAPTCHA unless it thinks the submission is spam. In my experience, it's been 100% effective, although the actual effectiveness can vary depending on your site, of course.

It's got a heck of a lot of interfaces as well. Just for reference (and future searchers) here is the URL:

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