Add Custom Field for WooCommerce Cart

action_add_referer_field.php

/**
 * Add a custom text input field to the product page
 * 從 URL 參數取得 referer ID，並顯示於表單欄位。
 */
function action_add_referer_field() { ?>
    <div class="referer-field-wrap">
        <label for="referer-field">轉介 ID</label>
        <input type="text" name='referer-field' id='referer-field' value='<?php echo $_GET['referer'] ?>'>
    </div>
<?php }
add_action( 'woocommerce_before_add_to_cart_button', 'action_add_referer_field' );

action_checkout_create_order_line_item.php

/**
 * Add custom meta to order
 */
function action_checkout_create_order_line_item( $item, $cart_item_key, $values, $order ) {
    // 如果購物車裡面有指定 'referer_id' 項目的值，就把該項目加入 order item meta data（保存於資料庫中）。
    // 你將能夠在 wp_woocommerce_order_itemmeta 資料表中找到 'referer_id' 項目的名稱與值。
    if ( isset( $values['referer_id'] ) ) {
        $item->add_meta_data('referer_id', $values['referer_id'], true);
    }
    else {
        $item->add_meta_data('referer_id', '(無)', true);
    }
}
add_action( 'woocommerce_checkout_create_order_line_item', 'action_checkout_create_order_line_item', 10, 4 );

filter_add_cart_item_data.php

/**
 * Add custom cart item data
 */
function filter_add_cart_item_data( $cart_item_data, $product_id, $variation_id ) {
    if( isset( $_POST['referer-field'] ) ) {
        $cart_item_data['referer_id'] = sanitize_text_field( $_POST['referer-field'] );
    }
    else {
        $cart_item_data['referer_id'] = '(無)';	
    }	
    return $cart_item_data;
}
add_filter( 'woocommerce_add_cart_item_data', 'filter_add_cart_item_data', 10, 3 );