Email Commenters When Someone Replies (Opt-In)

$wpide_reply_checkbox = function () {
    echo '<p class="comment-form-wpide-notify"><label><input type="checkbox" name="wpide_notify_replies" value="1"> Email me when someone replies to my comment</label></p>';
};

add_action( 'comment_form_after_fields', $wpide_reply_checkbox );
add_action( 'comment_form_logged_in_after', $wpide_reply_checkbox );

$wpide_notify_parent = function ( $comment ) {
    $comment = get_comment( $comment );

    if ( ! $comment || ! $comment->comment_parent || '1' !== (string) $comment->comment_approved || get_comment_meta( $comment->comment_ID, 'wpide_reply_notified', true ) ) {
        return;
    }

    $parent = get_comment( $comment->comment_parent );

    if ( ! $parent || ! get_comment_meta( $parent->comment_ID, 'wpide_notify_replies', true ) || ! is_email( $parent->comment_author_email ) || $parent->comment_author_email === $comment->comment_author_email ) {
        return;
    }

    update_comment_meta( $comment->comment_ID, 'wpide_reply_notified', 1 );

    wp_mail(
        $parent->comment_author_email,
        sprintf( 'New reply to your comment on "%s"', wp_specialchars_decode( get_the_title( $comment->comment_post_ID ), ENT_QUOTES ) ),
        sprintf(
            "Hi %s,\n\n%s replied to your comment:\n\n%s\n\nRead the conversation: %s",
            $parent->comment_author,
            $comment->comment_author,
            wp_trim_words( wp_strip_all_tags( $comment->comment_content ), 60 ),
            get_comment_link( $comment )
        )
    );
};

add_action( 'comment_post', function ( $comment_id, $approved ) use ( $wpide_notify_parent ) {
    if ( ! empty( $_POST['wpide_notify_replies'] ) ) {
        add_comment_meta( $comment_id, 'wpide_notify_replies', 1, true );
    }

    if ( 1 === $approved ) {
        $wpide_notify_parent( $comment_id );
    }
}, 10, 2 );

add_action( 'transition_comment_status', function ( $new_status, $old_status, $comment ) use ( $wpide_notify_parent ) {
    if ( 'approved' === $new_status && 'approved' !== $old_status ) {
        $wpide_notify_parent( $comment );
    }
}, 10, 3 );