Magento Category

category.php

public function categoryTravel($category, $level)
{
    $result = new Varien_Data_Collection;
    if (is_numeric($category)) {
        $category = Mage::getModel('catalog/category')->load($category);
    }

    $this->_categoryTravel($result, $level, $category);

    return $result;
}

protected function _categoryTravel($result, $level, $category)
{
    $result->addItem($category);
    if ($level === null || $category->getLevel() < $level) {
        $categoryChildren = $category->getChildrenCategories();
        foreach ($categoryChildren as $_category) {
            $this->_categoryTravel($result, $level, $_category);
        }
    }
}
// Use function
categoryTravel(5, 6); // Category id = 5, level = 6