get all categories and subcategories in magento

<?php 
$_categories = Mage::getBlockSingleton('catalog/navigation'); 
foreach ($_categories->getStoreCategories() as $_category) { 
$category = Mage::getModel('catalog/category'); 
$category->load($_category->getId()); 
$subcategories = explode(',', $category->getChildren()); 
?> 
<dl> 
<dt><?php echo $this->htmlEscape($_category->getName()); ?></dt> 
<dd> 
<ol> 
<?php 
foreach ($subcategories as $subcategoryId) { 
$category->load($subcategoryId); 
echo '<li><a href="' . $category->getURL() . '">' . $category->getName() . '</a></li>'; 
} 
?> 
</ol> 
</dd> 
</dl> 
<?php

} 
?>