Get product categories under a particular parent (Magento 1.x)

subs.php

<?php

function getSubs($product, $parentId)
{
    $cats = array();
    
    // get the product category ids
    $ids = $product->getCategoryIds();
    
    // get all children of the parent $catId
    $children = Mage::getModel('catalog/category')->getCategories($parentId, 0, false, true);
    foreach ($children as $category) {
        
        // if the child is in the product categories,
        // include it
        if (in_array($category->getId(), $ids)) {
            $cats[] = $category;
        }
    }
    
    return $cats;
}