Conclusion

After you're happy with your CAPTCHA strength, you need to setup the check in your form submission page to check that the person filling out the form has entered the correct code. This is very easy to do and all it takes is a simple if statement:

if ($_SESSION['captcha'] != $_POST['captcha']) {
	echo "The image verification failed.<br />\n";
	return false;
}

You should keep in mind however that if you make your CAPTCHA very strong then sometimes a person might not be able to work out what the text says and may accidently type a letter wrong so if the form is one that they may take a while to fill out like a contact form, you should redisplay their form with the warning rather then taking them to the results page and then going back to a form that they will have to fill out all over again.

A CAPTCHA can be a very effective way to reduce the amount of spam submitted to your site, however you should also be aware of the limitations of a CAPTCHA when choosing if you should implement one on your site.