Magento: Find subcategories for a root category

We can use the below code to find all child categories for a particular category

in Magento

 

<?php

//get category model

    $category_model = Mage::getModel('catalog/category'); 

 //$categoryid for which we have to find the child categories

    $_category = $category_model->load($categoryid); 

 //array that contains all child categories id

        $all_child_categories = $category_model->getResource()->getAllChildren($_category);

?>