functions.php

<?php
// Function to get the referrer user ID
function get_referrer_user_id($rafID) {
    // Check if the referrer is a guest using email
    if (filter_var($rafID, FILTER_VALIDATE_EMAIL)) {
        return $rafID;
    }

    // Retrieve the referrer user ID based on the custom meta field 'gens_referral_id'
    $gens_users = get_users(array(
        "meta_key" => "gens_referral_id",
        "meta_value" => $rafID,
        "number" => 1,
        "fields" => "ID"
    ));

    // Return the user ID if found, otherwise return false
    if (is_array($gens_users) && !empty($gens_users)) {
        return $gens_users[0];
    } else {
        return false;
    }
}

// Function to get the subscription level of a user
function get_user_subscription_level($user_id) {
    // Return 'none' if no user ID is provided
    if (!$user_id) {
        return 'none';
    }

    // Get the user's subscriptions
    $subscriptions = wcs_get_users_subscriptions($user_id);
    foreach ($subscriptions as $subscription) {
        // Check if the subscription is active or on hold
        if ($subscription->has_status(array('active', 'on-hold'))) {
            // Get the product ID from the subscription
            $product_id = $subscription->get_items()[0]->get_product_id();
            // Return the subscription level based on the product ID
            if ($product_id == 'basic_product_id') {
                return 'basic';
            } elseif ($product_id == 'middle_product_id') {
                return 'middle';
            } elseif ($product_id == 'advanced_product_id') {
                return 'advanced';
            }
        }
    }
    // Return 'none' if no active subscriptions are found
    return 'none';
}

// Function to get the referral discount based on the subscription level
function get_referral_discount($subscription_level) {
    switch ($subscription_level) {
        case 'basic':
            return 5; // 5% discount for basic level
        case 'middle':
            return 10; // 10% discount for middle level
        case 'advanced':
            return 15; // 15% discount for advanced level
        default:
            return 0; // No discount
    }
}

// Hook into the WooCommerce filter to apply the referral discount
add_filter('prefix_coupon_amount', 'apply_referral_discount', 10, 2);
function apply_referral_discount($amount, $order_id) {
    // Get the order
    $order = wc_get_order($order_id);
    // Get the referrer ID from the order meta
    $rafID = esc_attr($order->get_meta('_raf_id', true));
    // Get the referrer user ID using the function defined earlier
    $referrer_user_id = get_referrer_user_id($rafID);

    // Get the subscription level of the referrer
    $subscription_level = get_user_subscription_level($referrer_user_id);
    // Get the discount amount based on the subscription level
    $discount = get_referral_discount($subscription_level);

    // Apply the discount to the amount
    $amount = $amount - ($amount * ($discount / 100));
    return $amount;
}