add_action( 'comment_form', 'add_hidden_fields' );
        function add_hidden_fields() {
            $mathvalue0 = rand(2, 15);
            $mathvalue1 = rand(2, 15);
      echo '<div><b>IMPORTANT!</b> To be able to proceed, you need to solve the following simple math (so we know that you are a human) :-) <br/><br/>';
            echo "What is $mathvalue0 + $mathvalue1 ?<br/>";            
            echo '<input type="text" name="mathvalue2" value="" />';
            echo '</div>';
            echo '<div style="display:none">Please leave these two fields as-is: ';
            echo "<input type='text' name='mathvalue0' value='$mathvalue0' />";
            echo "<input type='text' name='mathvalue1' value='$mathvalue1' />";
            echo '</div>';
        }
        add_filter( 'preprocess_comment', 'preprocess_comment' );
        function preprocess_comment( $commentdata ) {
            check_hidden_fields();
            return $commentdata;
        }
                function check_hidden_fields() {
            // Get values from POST data
            $val0 = '';
            $val1 = '';
            $val2 = '';
            if ( isset( $_POST['mathvalue0'] ) ) {
                $val0 = $_POST['mathvalue0'];
            }
            if ( isset( $_POST['mathvalue1'] ) ) {
                $val1 = $_POST['mathvalue1'];
            }
            if ( isset( $_POST['mathvalue2'] ) ) {
                $val2 = $_POST['mathvalue2'];
            }
            
            // Check values
            if ( ( $val0 == '' ) || ( $val1 == '' ) || ( intval($val2) != (intval($val0) + intval($val1)) ) ) {
                // Die and return error 403 Forbidden
                wp_die( 'Bye Bye, SPAMBOT!', '403 Forbidden', array( 'response' => 403 ) );
            }
        }