你的模块需要辅助类的原因是 system.xml 文件中的 module 参数:

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.这反过来将类 id 补全到 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.

到目前为止还算简单，但对于某些人来说，这里可能有点混乱.
如果 Magento 没有找到辅助类前缀，它会尝试通过在类 ID 前面加上 mage_ 并附加 _helper 来弥补.
所以这给你 mage_cashondelivery_helper 作为类前缀，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，无法找到，因此您遇到了异常.

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 类之外，您还需要将类组映射添​​加到您的 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>

这个类组到类前缀映射是你所缺少的.

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