You should be running the final version of the Book Reviews plugin created in Chapter 4, The Power of Custom Post Types, and should have already followed the Sending email notifications upon new submissions recipe. Alternatively, you can get the resulting files from the code bundle (Chapter 4/ch4-book-reviews/ch4-book-reviews-v11.php and Chapter 6/ch6-book-review-user-submission/ch6-book-review-user-submission-v3.php) and rename ch6-book-review-user-submission-v3.php to ch6-book-review-user-submission.php before starting the recipe.

The GD and FreeType libraries need to be installed and activated in your development web server's PHP installation to be able to generate a CAPTCHA image. Most of the pre-packaged local web servers listed in the Installing a web server on your computer recipe from Chapter 1, Preparing a Local Development Environment, come with these libraries activated. They are also commonly enabled on most hosted web servers, but you should still mention that they are required when distributing a plugin containing the following code to a larger audience.