How to get category id from product id in magento

How to get category id from product id in magento

<?php
$product = Mage::getModel('catalog/product')->load(6);

$cats = $product->getCategoryIds();
foreach ($cats as $category_id) {
    $_cat = Mage::getModel('catalog/category')->load($category_id) ;
    echo $_cat->getName();
}
?>