WooCommerce after cart validation apply a voucher code as coupon discount

Question

I'm building some functionality in WooCommerce that allows users to input an externally generated voucher code when adding a product to the cart. This voucher code is then checked to see if it's been previously used and if not I want to apply a generic coupon code to the order.

I have the validation working using the below code

// Define field label name
function get_field_label_name()
{
    return __("Voucher");
}

// Add a custom product note below product meta in single product pages
add_action('woocommerce_before_single_variation', 'my_product_custom_field', 100);
function my_product_custom_field()
{

    $voucher_check = get_field('voucher');
    $voucher_text = get_field('voucher_text');

    if ($voucher_check) {
        echo '<div class="vouchers_single">';
        echo '<p>' . $voucher_text . '</p>';


        woocommerce_form_field('custom_field1', array(
            'type'        => 'text',
            'class'       => array('my-field-class form-row-wide'),
            'label'       => get_field_label_name(),
            'placeholder' => __("The field placeholder…"),
            'required'    => true, // Or false
        ), '');

        echo '</div>';
    }
}

// Check if the custom field value is unique
add_filter('woocommerce_add_to_cart_validation', 'wc_add_on_feature', 20, 3);
function wc_add_on_feature($passed, $product_id, $quantity)
{
    if (isset($_POST['custom_field1']) && !empty($_POST['custom_field1'])) {
        global $wpdb;

        $label = get_field_label_name();
        $value = sanitize_text_field($_POST['custom_field1']);

        // Check if value exits already
        $result = $wpdb->get_var("
            SELECT COUNT(meta_value) FROM {$wpdb->prefix}woocommerce_order_itemmeta
            WHERE meta_key LIKE '$label' AND meta_value LIKE '$value'
        ");

        // If it exist we don't allow add to cart
        if ($result > 0) {
            // Display an error notice
            wc_add_notice(sprintf(__('This "%s" input already exist. Please choose another one…'), $value), 'error');
            $passed = false;
        }
    }
    return $passed;
}

I also have this code snippet to apply the code

add_action('woocommerce_before_cart_table', 'ts_apply_discount_to_cart');

function ts_apply_discount_to_cart()
{

    $order_total = WC()->cart->get_subtotal();

    if ($order_total > 250) {
        $coupon_code = 'retrofitwest';
        if (!WC()->cart->add_discount(sanitize_text_field($coupon_code))) {
            wc_print_notices();
        }
    }
}

But what filter do I need to use to apply the coupon once the custom_field validation is passed?

Any help would be greatly appreciated.

Answer 1

In the following, I have revisited mostly all your code:

• When making an SQL query, always use prepare() function to avoid SQL injections issues. I have added that code in a separated function just for that query.
• Added a Function to save the voucher code on add to cart as custom cart item data.
• I have renamed "custom_field1" as "voucher_code".
• I have changed your last function completely (If the voucher code has been added as custom cart item data, the code is applied as a coupon, if not 'retrofitwest' coupon code is applied instead).

The code (untested as you are using some ACF fields):

// Define field label name
function get_field_label_name()
{
    return __("Voucher");
}

// Check (securely) if a voucher code has been used before
function check_submitted_voucher_code( $value ) {
    global $wpdb;

    return $wpdb->get_var( $wpdb->prepare("
        SELECT COUNT(meta_value) FROM {$wpdb->prefix}woocommerce_order_itemmeta
        WHERE meta_key = '%s' AND meta_value = '%s'
    ", get_field_label_name(), $value ) );
}

// Add a custom product note below product meta in single product pages
add_action('woocommerce_before_single_variation', 'my_product_custom_field', 100);
function my_product_custom_field() {
    if ( get_field('voucher') ) {
        echo '<div class="vouchers_single">
        <p>' . get_field('voucher_text') . '</p>';

        woocommerce_form_field('voucher_code', array(
            'type'        => 'text',
            'class'       => array('my-field-class form-row-wide'),
            'label'       => get_field_label_name(),
            'placeholder' => __("The field placeholder…"),
            'required'    => true, // Or false
        ), '');

        echo '</div>';
    }
}

// Voucher code validation
add_filter('woocommerce_add_to_cart_validation', 'wc_add_on_feature' );
function wc_add_on_feature( $passed ){
    if (isset($_POST['voucher_code']) && ! empty($_POST['voucher_code'])) {
        $value = sanitize_text_field($_POST['voucher_code']);

        // Check if value exits already avoiding add to cart
        if ( check_submitted_voucher_code( $value ) > 0) {
            // Display an error notice
            wc_add_notice(sprintf(__('This "%s" input already exist. Please choose another one…'), $value), 'error');
            $passed = false;
        }
    }
    return $passed;
}

// Save submitted valid voucher code
add_filter('woocommerce_add_cart_item_data', 'save_custom_cart_item_data' );
function save_custom_cart_item_data( $cart_item_data ){
    if (isset($_POST['voucher_code']) && ! empty($_POST['voucher_code'])) {
        // Set the custom data in the cart item
        $cart_item_data['voucher_code'] = sanitize_text_field($_POST['voucher_code']);
    }
    return $cart_item_data;
}

// Apply the discount
add_action('woocommerce_before_calculate_totals', 'applied_cart_discount');
function applied_cart_discount( $cart ){
    if ( is_admin() && ! defined('DOING_AJAX') )
        return;

    $voucher_code  = ''; // Initializing

    // Loop through cart items
    foreach( $cart->get_cart() as $item ) {
        if( isset($item['voucher_code']) ) {
            $voucher_code = $item['voucher_code'];
            break;
        }
    }

    if ( $cart->get_subtotal() > 250 ) {
        $coupon_code = ! empty($voucher_code) ? $voucher_code : 'retrofitwest';
        $cart->apply_coupon($coupon_code);
    }
}

Code goes in functions.php file of your child theme (or in a plugin). It should work.