PHP Form Wizard, does not have the feature to add a "CAPTCHA" to your form, but there are various sources that provide this service.
An easy to use one would be reCAPTCHA by Google, because it's JavaScript embedded, meaning there is little work for you to do in order to add this security to your registration form.
Since there is no work from your server side to add this, Google uses their servers for this to work, follow the steps bellow to successfully embed reCAPTCHA on your form.
1. Register a key for your website(s) with Google.
2. Embed the code bellow(in your form), but edit your_public_key with the key you just received from finishing step 1.
<script type="text/javascript"
src="http://www.google.com/recaptcha/api/challenge?k=your_public_key">
</script>
<noscript>
<iframe src="http://www.google.com/recaptcha/api/noscript?k=your_public_key"
height="300" width="500" frameborder="0"></iframe><br>
<textarea name="recaptcha_challenge_field" rows="3" cols="40">
</textarea>
<input type="hidden" name="recaptcha_response_field"
value="manual_challenge">
</noscript>
If done right, you should now see a CAPTCHA prompt appear on your form.
For more information you can check their official website here.
