[site-reviews] send email to author (or to a custom email) after review is approved

/**
 * Sends an email to the review author (or a custom email) when their review has been approved.
 * Paste this in your active theme's functions.php file.
 *
 * @param string $oldStatus
 * @param string $newStatus
 * @param \WP_Post $post
 * @return void
 * @action transition_post_status
 */
add_action('transition_post_status', function ($newStatus, $oldStatus, $post) {
    if (in_array($oldStatus, ['new', $newStatus])
        || 'site-review' != $post->post_type
        || 'publish' != $post->post_status) {
        return;
    }
    $review = apply_filters('glsr_get_review', null, $post->ID);

    // Start: change these values as needed
    $email = $review->email;
    $subject = "Your review has been published!";
    $message = "Thank you so much for your review! Just letting you know that it's now live on our website.";
    // End

    $metaKey = '_sent_approved_notification';
    $alreadySentNotification = get_post_meta($post->ID, $metaKey, true);

    if (!empty($email) && !$alreadySentNotification) {
        if ($sent = wp_mail($email, $subject, $message)) {
            update_post_meta($post->ID, $metaKey, true);
        } else {
            apply_filters('glsr_log', null, 'The notification to <'.$email.'> was not sent. Please verify that your server is able to send emails correctly through WordPress.');
        }
    }
}, 10, 3);