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<?php
$categoryDefault = array(
    'is_active'         => 1,
    'include_in_menu'   => 0,
    'available_sort_by' => null,
    'default_sort_by'   => null,
);
$mainCategory = 2;

$categoryPath = Mage_Catalog_Model_Category::TREE_ROOT_ID . '/' . $mainCategory;

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

$categoryModel->addData($categoryDefault);
$categoryModel->setName('name');
$categoryModel->setPath($categoryPath);
$categoryModel->setAttributeSetId($categoryModel->getDefaultAttributeSetId());
$categoryModel->setStoreId(0);
$categoryModel->save();