* List available attributes on the product page in a drop-down selection
 */
function list_attributes_on_product_page() {
    global $product;
    $attributes = $product->get_attributes();

    if ( ! $attributes ) {
        return;
    }

    //from original script, but here we want to use it for variable products
    /*if ($product->is_type( 'variable' )) {
        return;
    }*/

    echo '<div style="padding-bottom:15px;">';

    foreach ( $attributes as $attribute ) {

        //If product is variable, and attribute is used for variation: woocommerce already handle this input - so it can also be used with attributes of simple products (not variables)
    if($product->is_type( 'variable' ) && $attribute['variation']) {
        continue;
    }

        //get taxonomy for the attribute - eg: Size
        $taxonomy = get_taxonomy($attribute['name']);

        //get terms - eg: small
        $options = wc_get_product_terms( $product->get_id(), $attribute['name'], array( 'fields' => 'all' ) );
        $label = str_replace('Product ', '', $taxonomy->label);
        //display select input
        ?>
        <div style="padding-bottom:8px;">
            <label for="attribute[<?php echo $attribute['id']; ?>]"><?php echo $label; ?></label>
            <br />
            <!-- add required attribute or not, handle default with "selected" attribute depending your needs -->
            <select name="attribute[<?php echo $attribute['id']; ?>]" id="attribute[<?php echo $attribute['id']; ?>]">
                <option value disabled selected>Choose an option</option>
                <?php foreach ( $options as $pa ): ?>
                    <option value="<?php echo $pa->name; ?>"><?php echo $pa->name; ?></option>
                <?php endforeach; ?>
            </select>
        </div>
        <?php
    }

    echo '</div>';
}
add_action('woocommerce_before_add_to_cart_button', 'list_attributes_on_product_page');

/**
 * Add selected attributes to cart items
 */
add_filter('woocommerce_add_cart_item_data', 'add_attributes_to_cart_item', 10, 3 );
function add_attributes_to_cart_item( $cart_item_data, $product_id, $variation_id ) {
    $attributes = $_POST['attribute'] ?? null;

    if (empty( $attributes ) ) {
        return $cart_item_data;
    }

    $cart_item_data['attributes'] = serialize($attributes);

    return $cart_item_data;
}

/**
 * Display attributes in cart
 */
add_filter( 'woocommerce_get_item_data', 'display_attributes_in_cart', 10, 2 );
function display_attributes_in_cart( $item_data, $cart_item ) {
    if ( empty( $cart_item['attributes'] ) ) {
        return $item_data;
    }

    foreach (unserialize($cart_item['attributes']) as $attributeID => $value) {
        $attribute = wc_get_attribute($attributeID);
        $item_data[] = array(
            'key'     => $attribute->name,
            'value'   => $value,
            'display' => '',
        );
    }

    return $item_data;
}

/**
 * Add attribute data to order items
 */
add_action( 'woocommerce_checkout_create_order_line_item', 'add_attributes_to_order_items', 10, 4 );
function add_attributes_to_order_items( $item, $cart_item_key, $values, $order ) {
    if ( empty( $values['attributes'] ) ) {
        return;
    }

    foreach (unserialize($values['attributes']) as $attributeID => $value) {
        $attribute = wc_get_attribute($attributeID);
        $item->add_meta_data( $attribute->name, $value );
    }
}