您的模块需要帮助程序类的原因是 system.xml 文件中的module自变量:

The reason your module needs a helper class is the module argument in the system.xml file:

<cashondelivery translate="label" module="cashondelivery">

Magento将module参数传递给Mage::helper()工厂方法.依次将类ID补全为cashondelivery/data.

Magento passes the module argument to the Mage::helper() factory method. This in turn completes the class id to cashondelivery/data.

Mage::helper('cashondelivery');
// identical to Mage::helper('cashondelivery/data');

现在，Magento通过查找缺少的节点global/helpers/cashondelivery/class来检查要使用的类前缀.

Now Magento checks for the class prefix to use by looking for the node global/helpers/cashondelivery/class which is missing.

到目前为止，它还很简单，但是在这里可能有些让人困惑.
如果Magento找不到助手类的前缀，它会尝试通过在类ID的前面加上mage_并附加_helper来弥补.
因此，这将为您提供mage_cashondelivery_helper作为类前缀，并为mage_cashondelivery_helper_data作为完整的类名.

Rather straightforward so far, but here is where it might be a little confusing for some.
If Magento doesn't find a helper class prefix, it tries to makes one up by prefixing the class id with mage_ and appending _helper.
So this gives you mage_cashondelivery_helper as the class prefix, and mage_cashondelivery_helper_data as the full class name.

自动加载器将其转换为 Mage/Cashondelivery/Helper/Data.php ，找不到该文件，因此您遇到的异常.

The autoloader turns this into Mage/Cashondelivery/Helper/Data.php, which can't be found, and hence the exception you are experiencing.

除了创建Companyname_Cashondelivery_Helper_Data类外，还需要将类组映射添​​加到config.xml文件中，如下所示:

Besides creating the Companyname_Cashondelivery_Helper_Data class, you need to add the class group mapping to your config.xml file as follows:

<global>
  <helpers>
    <cashondelivery>
      <class>Companyname_Cashondelivery_Helper</class>
    </cashondelivery>
  </helpers>
</global>

您所缺少的只是该类组到类前缀的映射.

This class group to class prefix mapping is all you are missing.