How to get all category tree node in Magento

gistfile1.php

<?php
// Usually, you can get category tree from category helper
$helper = Mage::helper('catalog/category');
$nodes = $helper->getStoreCategories();
// return Varien_Data_Tree_Node_Collection
//        via Mage_Catalog_Model_Resource_Category


// However, this get method return active category only.
// Most of the samples are for collection of the category.
// This is for a tree node not a normal collection.
// We can get all category tree using the code below:

$parent = Mage::app()->getStore()->getRootCategoryId();
$tree = Mage::getResourceModel('catalog/category_tree');

$nodes = $tree->loadNode($parent)
  ->loadChildren($recursionLevel)
  ->getChildren();

$tree->addCollectionData(null, false, $parent, true, false);

// Now, you can use $nodes as category tree
foreach($nodes as $category){
  // Do something
}
?>