Create the Plugin Skeleton – “Hello Woo Checkout”

In this chapter, we’ll create a real WordPress plugin that WooCommerce can use. By the end, you’ll have:

• A proper plugin folder and main file
• A clean naming convention to follow
• A working “Hello from our plugin” message on the checkout page

1. Choose a Plugin Name & Slug

Let’s standardize things from the start.

• Plugin Name (human-readable): Simple Discount Rules for WooCommerce
• Plugin Slug (machine-friendly): simple-discount-rules

We’ll use this slug consistently for:

• Folder name
• Main plugin file
• Function prefixes, constants, etc.

Rule of thumb:
Use kebab-case (lowercase, words separated by hyphens) for folder/file names,
and snake_case with a short prefix for functions (e.g., sdr_).

2. Plugin Folder Structure

Inside your WordPress install, go to:
wp-content/plugins/simple-discount-rules/

Create a folder named:
simple-discount-rules

Inside it, create this basic structure:
wp-content/
plugins/
simple-discount-rules/
├── simple-discount-rules.php ← Main plugin file (required)
├── includes/ ← For additional PHP files (future use)
└── assets/ ← For CSS / JS / Images (optional)

For now, only simple-discount-rules.php is required. The other folders are just planning ahead.

3. Create the Main Plugin File

Open simple-discount-rules.php in your code editor and add this:

Simple Discount Rules Plugin Example

This code shows how to create a basic WooCommerce plugin that displays a custom message on the checkout page.

<?php 
/**
 * Plugin Name:       Simple Discount Rules for WooCommerce
 * Plugin URI:        https://example.com/simple-discount-rules
 * Description:       A step-by-step tutorial plugin that adds custom discount rules to WooCommerce checkout.
 * Version:           1.0.0
 * Author:            Your Name
 * Author URI:        https://example.com
 * Text Domain:       simple-discount-rules
 * Requires at least: 6.0
 * Requires PHP:      7.4
 */

if ( ! defined( 'ABSPATH' ) ) {
    exit; // Exit if accessed directly.
}

/**
 * Basic constants.
 */
define( 'SDR_PLUGIN_VERSION', '1.0.0' );
define( 'SDR_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
define( 'SDR_PLUGIN_URL', plugin_dir_url( __FILE__ ) );

/**
 * Simple check: only run if WooCommerce is active.
 */
function sdr_is_woocommerce_active() {
    return class_exists( 'WooCommerce' );
}

/**
 * Force Classic Checkout (disable WooCommerce Block Checkout).
 */
add_filter( 'woocommerce_blocks_is_checkout_block_enabled', '__return_false', 9999 );
add_filter( 'woocommerce_blocks_is_cart_block_enabled', '__return_false', 9999 );

/**
 * Remove block-theme support.
 */
add_action( 'wp', function() {
    if ( function_exists( 'remove_theme_support' ) ) {
        remove_theme_support( 'woocommerce-block-theme' );
    }
}, 20 );

/**
 * Output a simple message on the WooCommerce checkout page.
 */
function sdr_checkout_hello_message() {

    if ( ! sdr_is_woocommerce_active() ) {
        return;
    }

    $message = __( 'Hello from Simple Discount Rules plugin! This message is coming from your custom WooCommerce extension.', 'simple-discount-rules' );

    echo '<p style="padding: 10px; background: #f5f5f5; border-left: 4px solid #2271b1; margin-bottom: 15px;">';
    echo esc_html( $message );
    echo '</p>';
}
add_action( 'woocommerce_before_checkout_form', 'sdr_checkout_hello_message', 5 );

Why this works: the add_filter( ‘woocommerce_blocks_is_checkout_block_enabled’, ‘__return_false’, 9999 ) line tells WooCommerce to stop using the block checkout UI, which then allows woocommerce_before_checkout_form (and other classic hooks) to run.

4. Activate the Plugin

Now go to your WordPress admin:

1. Navigate to Plugins → Installed Plugins
2. Find “Simple Discount Rules for WooCommerce”
3. Click Activate

It won’t do anything yet—but if activation works without errors, your skeleton is correct.

5. Test the checkout message

• Make sure WooCommerce is installed and active and you have at least one product.
• Add a product to the cart.
• Visit the Checkout page (/checkout/).
• You should see the styled message at the top of the checkout form:

If you still don’t see it:

• Clear browser and site caches, then reload.

• Confirm the checkout page contains the classic [woocommerce_checkout] shortcode (if you manually edited the page).

• Confirm plugin is active and WooCommerce is active.

  If you deactivate the plugin and refresh checkout:

  • The message should disappear
  • That confirms the hook lives inside your plugin.
    

7. Summary of What We’ve Done

By the end of Chapter 1, you now have:

• A proper plugin folder and file structure
• A clear naming convention (slug, file names, function prefix)
• A main plugin file with header and basic constants
• A working “Hello Checkout” message injected via a WooCommerce hook

In the next chapter, we’ll:

Add an admin settings page under WooCommerce so we can customize this     checkout message from the dashboard instead of hard-coding it in PHP.