How to validate recaptcha on comments form?

Question

I've added the following code to my function file to add the recaptcha, however I do not know how I can validate the captcha.

/**
* Add Captcha to Comments Form
 */
add_filter('comment_form', function() {
    echo '<div class="g-recaptcha" data-sitekey="'.GOOGLE_RECAPTCHA_SITEKEY.'"></div>';
});

Is there a filter for the submission of the comments form? I will be validating the captcha server side using the following:

https://github.com/google/recaptcha

Any advise on how to implement the captcha to the comments form would be appreciated.

Answer 1

There's a preprocess_comment filter that is run before the comment is inserted in the database.

You will have access to the comment's data:

add_filter( 'preprocess_comment' , 'wpse321083_process_recaptcha' );
function wpse321083_process_recaptcha( $commentdata ) {
    // Process recaptcha here
    return $commentdata;
}

Here's also a good article on SitePoint.com explaining how to implement this feature in your website.