Add product to WooCommerce cart & redirect to checkout via URL parameter

product-checkout.php

<?php

/**
 *  Add a direct-to-checkout functionality to WooCommerce.
 * 
 * 
 * When user follows the link with productcheckout parameter, it
 * clears the existing shopping cart and adds the specified product
 * to the cart (with optional quantity). You can add multiple products
 * separated by commas.
 * Then it redirects the user directly to the WooCommerce checkout page.
 * You can also add coupon with the optional coupon parameter
 * 
 * To use this functionality, you would construct a URL like this:
 * https://yourwebsite.com/?productcheckout=123&quantity=2 
 * Replace '123' with the actual WooCommerce product ID.
 * The 'quantity' parameter is optional and defaults to 1 if not provided.
 * 
 * Installation
 * Save this script in your WordPress installation's 
 * wp-content/mu-plugins/product-checkout.php directory.
 * Note that mu-plugins are always active, so you don't need to worry about
 * plugin activation.
 */

<?php
// Exit if accessed directly
if (!defined('ABSPATH')) {
    exit;
}

add_action('template_redirect', 'custom_add_to_cart_redirect', 1);

function custom_add_to_cart_redirect() {
    // Check if the 'productcheckout' is present in the URL
    if (isset($_GET['productcheckout'])) {
        $product_ids = explode(',', sanitize_text_field($_GET['productcheckout']));

        error_log('Custom redirect triggered for products: ' . implode(', ', $product_ids));

        WC()->cart->empty_cart();

        foreach ($product_ids as $product_id) {
            $product_id = absint($product_id);
            if ($product_id) {
                WC()->cart->add_to_cart($product_id, isset($_GET['quantity']) ? absint($_GET['quantity']) : 1);
            }
        }

        // Check if there is a coupon to apply
        if (isset($_GET['coupon'])) {
            $coupon_code = sanitize_text_field($_GET['coupon']);
            WC()->cart->apply_coupon($coupon_code);
            error_log('Coupon applied: ' . $coupon_code);
        }

        wp_safe_redirect(wc_get_checkout_url());
        exit;
    }
}