Call a public function within a FieldType file, from a variables files
From what I understand about plugin development on Craft, this isn’t possible. The idea of the variables file is to provide methods which are accessible from within your plugin or users templates.

The idea being that the methods you define here are actually references to your service methods (Taken from Craft docs) 


  Many of your template variable methods will likely be simple wrappers for your service API methods. It’s important to remember not to expose state-changing methods, however.


So from that I would actually create a service class for your plugin and access the methods in your getInputHtml function from that.

For example:

services/MyPlugin_PluginService.php

&lt;?php
namespace Craft;

class MyPlugin_PluginService extends BaseApplicationComponent
{
 public function helloWorld()
 {
   return ‘Hello World’;    
  }
}


In your getInputHtml function

$testing = craft()-&gt;myPlugin_plugin-&gt;helloWorld();


Call the same method in your templates

Add this to your variables/MyPluginVariable.php file to call the method in your templates:

public function sayHello() {
  return craft()-&gt;myPlugin_plugin-&gt;helloWorld();
}


Then you could call the service method in your templates like so:

{{ craft.myPlugin.sayHello() }}


Hope that helps!