The Cloudways blog has a tutorial posted helping you add CAPTCHA functionality of your Symfony 3 form handling. In this case they're adding the Google reCAPTCHA handling to a registration form for users of your system.

Websites use different techniques and methods to validate contact, login and user registration forms. Some opt for custom validation methods while a large majority deploy third-party validation techniques.

Google provides a well known and proven validation tool known as reCAPTCHA. In this tutorial, I will demonstrate how to add Google reCAPTCHA to a Symfony 3 registration form. Hence, if you’re looking for a Recaptcha Symfony union, you’ve got it!

They start by helping you set up a Cloudways PHP application and install the latest version of the Symfony 3 framework on it as a sample application. They then walk you through the steps required to create a reCAPTCHA application and hooking it into your Cloudways server's hostname. With that set up they move over into the Symfony application, generating the User entity via Doctrine and creating the matching view/controller to handle the reCAPTCHA output. The registration form is then created and the handling is added to verify the CAPTCHA value submitted against the Google servers.