Set Automatic Notifications for Out-of-Stock Products in WooCommerce

Code

// Add a custom field to WooCommerce product data for enabling back in stock notifications
function wp_dudecom_add_back_in_stock_field() {
    global $post;
    echo '<div class="options_group">';
    woocommerce_wp_checkbox( array(
        'id'            => '_enable_back_in_stock_notifications',
        'label'         => __('Enable Back In Stock Notifications', 'woocommerce'),
        'description'   => __('Enable this to notify customers when this product is back in stock.', 'woocommerce'),
    ));
    echo '</div>';
}
add_action('woocommerce_product_options_inventory_product_data', 'wp_dudecom_add_back_in_stock_field');

// Save the custom field value
function wp_dudecom_save_back_in_stock_field($post_id) {
    $enable_back_in_stock = isset($_POST['_enable_back_in_stock_notifications']) ? 'yes' : 'no';
    update_post_meta($post_id, '_enable_back_in_stock_notifications', $enable_back_in_stock);
}
add_action('woocommerce_process_product_meta', 'wp_dudecom_save_back_in_stock_field');

// Notify customers when a product is back in stock
function wp_dudecom_notify_back_in_stock($product_id) {
    $product = wc_get_product($product_id);
    if ($product->is_in_stock() && 'yes' === get_post_meta($product_id, '_enable_back_in_stock_notifications', true)) {
        $subscribers = get_post_meta($product_id, '_back_in_stock_subscribers', true);
        if (!empty($subscribers)) {
            foreach ($subscribers as $subscriber_email) {
                wp_mail(
                    $subscriber_email,
                    __('Product Back In Stock', 'woocommerce'),
                    sprintf(__('The product %s is back in stock. You can purchase it now.', 'woocommerce'), $product->get_name())
                );
            }
            delete_post_meta($product_id, '_back_in_stock_subscribers');
        }
    }
}
add_action('woocommerce_product_set_stock_status', 'wp_dudecom_notify_back_in_stock', 10, 1);

// Add a form for customers to subscribe to back in stock notifications
function wp_dudecom_back_in_stock_form() {
    global $product;
    if (!$product->is_in_stock() && 'yes' === get_post_meta($product->get_id(), '_enable_back_in_stock_notifications', true)) {
        echo '<form method="post" id="back-in-stock-form">';
        echo '<input type="email" name="back_in_stock_email" placeholder="Enter your email" required />';
        echo '<button type="submit">' . __('Notify Me', 'woocommerce') . '</button>';
        echo '</form>';
    }
}
add_action('woocommerce_single_product_summary', 'wp_dudecom_back_in_stock_form', 35);

// Handle form submission and save subscriber email
function wp_dudecom_handle_back_in_stock_form() {
    if (isset($_POST['back_in_stock_email']) && is_email($_POST['back_in_stock_email'])) {
        global $product;
        $email = sanitize_email($_POST['back_in_stock_email']);
        $subscribers = get_post_meta($product->get_id(), '_back_in_stock_subscribers', true);
        if (empty($subscribers)) {
            $subscribers = array();
        }
        if (!in_array($email, $subscribers)) {
            $subscribers[] = $email;
            update_post_meta($product->get_id(), '_back_in_stock_subscribers', $subscribers);
            wc_add_notice(__('You will be notified when the product is back in stock.', 'woocommerce'), 'success');
        } else {
            wc_add_notice(__('You are already subscribed for back in stock notifications.', 'woocommerce'), 'notice');
        }
    }
}
add_action('wp', 'wp_dudecom_handle_back_in_stock_form');