WooCommerce: Allow adding multiple products to the cart via the add-to-cart query string

$product_id = 55;
$url = esc_url_raw( add_query_arg( 'add-to-cart', $product_id, wc_get_checkout_url() ) );

function woocommerce_maybe_add_multiple_products_to_cart( $url = false ) {
    // Make sure WC is installed, and add-to-cart qauery arg exists, and contains at least one comma.
    if ( ! class_exists( 'WC_Form_Handler' ) || empty( $_REQUEST['add-to-cart'] ) || false === strpos( $_REQUEST['add-to-cart'], ',' ) ) {
        return;
    }

    // Remove WooCommerce's hook, as it's useless (doesn't handle multiple products).
    remove_action( 'wp_loaded', array( 'WC_Form_Handler', 'add_to_cart_action' ), 20 );

    $product_ids = explode( ',', $_REQUEST['add-to-cart'] );
    $count       = count( $product_ids );
    $number      = 0;

    foreach ( $product_ids as $id_and_quantity ) {
        // Check for quantities defined in curie notation (<product_id>:<product_quantity>)
        // https://dsgnwrks.pro/snippets/woocommerce-allow-adding-multiple-products-to-the-cart-via-the-add-to-cart-query-string/#comment-12236
        $id_and_quantity = explode( ':', $id_and_quantity );
        $product_id = $id_and_quantity[0];

        $_REQUEST['quantity'] = ! empty( $id_and_quantity[1] ) ? absint( $id_and_quantity[1] ) : 1;

        if ( ++$number === $count ) {
            // Ok, final item, let's send it back to woocommerce's add_to_cart_action method for handling.
            $_REQUEST['add-to-cart'] = $product_id;

            return WC_Form_Handler::add_to_cart_action( $url );
        }

        $product_id        = apply_filters( 'woocommerce_add_to_cart_product_id', absint( $product_id ) );
        $was_added_to_cart = false;
        $adding_to_cart    = wc_get_product( $product_id );

        if ( ! $adding_to_cart ) {
            continue;
        }

        $add_to_cart_handler = apply_filters( 'woocommerce_add_to_cart_handler', $adding_to_cart->get_type(), $adding_to_cart );

        // Variable product handling
        if ( 'variable' === $add_to_cart_handler ) {
            woo_hack_invoke_private_method( 'WC_Form_Handler', 'add_to_cart_handler_variable', $product_id );

        // Grouped Products
        } elseif ( 'grouped' === $add_to_cart_handler ) {
            woo_hack_invoke_private_method( 'WC_Form_Handler', 'add_to_cart_handler_grouped', $product_id );

        // Custom Handler
        } elseif ( has_action( 'woocommerce_add_to_cart_handler_' . $add_to_cart_handler ) ){
            do_action( 'woocommerce_add_to_cart_handler_' . $add_to_cart_handler, $url );

        // Simple Products
        } else {
            woo_hack_invoke_private_method( 'WC_Form_Handler', 'add_to_cart_handler_simple', $product_id );
        }
    }
}

// Fire before the WC_Form_Handler::add_to_cart_action callback.
add_action( 'wp_loaded', 'woocommerce_maybe_add_multiple_products_to_cart', 15 );


/**
 * Invoke class private method
 *
 * @since   0.1.0
 *
 * @param   string $class_name
 * @param   string $methodName
 *
 * @return  mixed
 */
function woo_hack_invoke_private_method( $class_name, $methodName ) {
    if ( version_compare( phpversion(), '5.3', '<' ) ) {
        throw new Exception( 'PHP version does not support ReflectionClass::setAccessible()', __LINE__ );
    }

    $args = func_get_args();
    unset( $args[0], $args[1] );
    $reflection = new ReflectionClass( $class_name );
    $method = $reflection->getMethod( $methodName );
    $method->setAccessible( true );

    $args = array_merge( array( $class_name ), $args );
    return call_user_func_array( array( $method, 'invoke' ), $args );
}

// Multiple products, multiple quantities per product.
$product_ids = '63833:2,221916:4';
$add_to_cart_url = esc_url_raw( add_query_arg( 'add-to-cart', $product_ids, wc_get_checkout_url() ) );

// Multiple products (default quantity of 1)
$product_ids = '63833,221916';
$add_to_cart_url = esc_url_raw( add_query_arg( 'add-to-cart', $product_ids, wc_get_checkout_url() ) );

// Normal add-to-cart URL
$product_ids = '63833';
$add_to_cart_url = esc_url_raw( add_query_arg( 'add-to-cart', $product_ids, wc_get_checkout_url() ) );