Add an extra anti spam comment form field in WordPress comment

anti-spam-comment-field.php

<?php
/**
 * Extra anti spam comment form field
 *
 * @package ndt_framework
 */

/**
 * Add anti spam comment form field
 * @param array $fields The default comment fields.
 */
add_filter( 'comment_form_default_fields', 'ndt_add_anti_spam_comment_field' );

function ndt_add_anti_spam_comment_field( $fields ) {
	// get random values for anti spam question
	$num1 = mt_rand(1, 10);
	$num2 = mt_rand(1, 10);

	// get operator by string
	$operator = apply_filters( 'ndt_anti_spam_operator', '+' );

	$total = $num1 + $num2;

	// validate operator
	if ( '*' == $operator ) {
		$total = $num1 * $num2;
	} else {
		$operator = '+';
	}

	$random_string = ( (string) $num1 ) . ' ' . $operator . ' ' . ( (string) $num2 ) . ' = ?';

	$req      = get_option( 'require_name_email' );
	$aria_req = ( $req ? " aria-required='true'" : '' );

	$fields['anti_spam'] = '<p class="comment-form-anti-spam">' . '<label for="anti-spam">' . __( 'Anti spam question', 'ndt_framework' ) . ( $req ? ' <span class="required">*</span>' : '' ) . '</label> ' .
		         '<input id="anti-spam" name="anti-spam" type="text" size="30"' . $aria_req . ' placeholder="' . $random_string . '" />' .
		         '<input type="hidden" name="rand-total" value="' . $total . '"></p>';

	return $fields;
}

/**
 * Validate before posting comment
 * @param int $comment_post_ID Post ID.
 */
add_action( 'pre_comment_on_post', 'ndt_validate_anti_spam_question' );

function ndt_validate_anti_spam_question( $comment_post_ID ) {
	// get user answer
	$answer       = ( isset($_POST['anti-spam']) )  ? trim(strip_tags($_POST['anti-spam'])) : null;
	// get correct answer
	$correct_answer = ( isset($_POST['rand-total']) ) ? $_POST['rand-total'] : null;

	$answer = (int) $answer;
	$correct_answer = (int) $correct_answer;

	// if user answer was wrong, exit immediately and print an error
	if ( $answer !== $correct_answer ) {
		wp_die( __('<strong>ERROR</strong>: your answer for anti spam question was wrong, please try again.'), 'ndt_framework' );
	}
}