Magento中类和方法的用法

Question

I have question about php classes and scope, Magento in specific. I get the following error, but other methods work,

Fatal error: Call to undefined function getChildCategories()

What syntax is required to make getChildCategories() method work work?

require_once '/var/www/myshop/app/Mage.php';

include "includes/mycompany_report_inventory.php";
Mage::app();

$category_collection = Mage::getModel('catalog/category')
    ->getCollection()
    ->addAttributeToSelect('*');

// instantiate and execute inventory report   
$mycompany_report_inventory = new mycompany_report_inventory();
$x = $mycompany_report_inventory->GetData($category_collection);

... in file includes/mycompany_report_inventory.php

class mycompany_report_inventory  {

   function GetData($category_collection){   

   $this->fnListHierarchy();

   }
   ...

    function fnListHierarchy()  {

      // this works:    
      $_categories = Mage::helper('catalog/category')->getStoreCategories();

      // var_dump this to see an associative array of all categories
      $categories = getChildCategories($_categories, true);

      // Run this to echo out a DOM
      renderCategoriesTree($_categories, true);
    }
}