This is common when referencing a missing helper. In many cases the class name itself is malformed or your shortname referencing it is incorrect, which is why Magento is looking for it in the path Mage_Giftcards_Helper_Data (see example 1 below). For the following examples I have set up a module called MyCompany_MyModule.

These are just a few of the many ways you can go awry with defining helpers:

Incorrect helper class alias:

I reference my helper as:

$helper = Mage::helper('mycompany');

I get the error:

Fatal error: Class 'Mage_Mycompany_Helper_Data' not found

What went wrong?

My helper class alias is defined as mymodule:

<helpers>
    <mymodule>
        <class>MyCompany_MyModule_Helper</class>
    </mymodule>
</helpers>

Changing my shortcode to Mage::helper('mymodule') produced the desired result.

Malformed class name

I reference my helper (correctly this time):

$helper = Mage::helper('mymodule');

I receive:

Fatal error: Class 'MyCompany_MyModule_Helper_Data' not found

What went wrong?

My class definition was missing "_Data":

class MyCompany_MyModule_Helper extends Mage_Core_Helper_Abstract
{


}

Helper/Custom:

This is similar to what happens when you try to refer to a helper class within a file not named "Data.php".

I reference my helper as:

$helper = Mage::helper('custom');

My module helper path was defined as:

<helpers>
    <mymodule>
        <class>MyCompany_MyModule_Helper</class>
    </mymodule>
</helpers>

I have a class file in app/code/local/MyCompany/MyModule/Helper/Custom.php

I get the error:

Fatal error: Class 'Mage_Custom_Helper_Data' not found

What went wrong?

You need to specify other helper classes in a particular module as sub-paths to your module helper's shortname. This is similar to how blocks and models work - but at the topmost level there is no subfolder.

I fix this error by referencing my Custom.php helper class file as such:

$helper = Mage::helper('mymodule/custom');

You configured your helper class like this.

<helpers>
    <rms>
        <class>NewHope_Rms_Helper</class>
    </rms>
</helpers>

So when you request you helper class with Mage::helper('rms'), Magento will trace your module, since it defines the helper alias name rms and then pick class name associate with it. So in this case, Magento has NewHope_Rms_Helper. The next step magento checks, whether you requested in particular helper class name. For example, if you were requested a helper class like Mage::helper(rms/example), then magento will append this example part to the class that already picked from your configuration and thus completes the class generation process(Here the class would be NewHope_Rms_Helper_Example). However if we didn't specify any specific helper name, then by default, Magento assumes this class name as Data. So in your case, Magento forms final helper class as NewHope_Rms_Helper_Data.

The name of the class is an indication to the location where that class should be defined. That means Magento will now look for your helper in app/code/local/NewHope/Rms/Helper/Data.php. If it is not present there, it will look in app/code/community/NewHope/Rms/Helper/Data.php. Then again in app/code/core directory and then again in lib directory. In your case, Magento can't find the helper class. So as a result it will throw an error.(Note location generated is NewHope\Rms and not NewHope\RmsUpdate).

So the configuration that you need here is

<helpers>
    <rms>
        <class>NewHope_RmsUpdate_Helper</class>
    </rms>
</helpers>

So this will generate a class NewHope_RmsUpdate_Helper_Data and thus first it will look in the location app/code/local/NewHope/RmsUpdate/Helper/Data.php. The file content should look like this.

<?php
class NewHope_RmsUpdate_Helper_Data extends Mage_Core_Helper_Abstract
{
}

Hope that will give you an idea on what is really happens in the backside