Magento

Ok, here is how you can add a category using Magento API:

// initialize magento environment for 'default' store
require_once 'app/Mage.php';
Mage::app('default');

//get a new category object
$category = Mage::getModel('catalog/category');
$category->setStoreId(0); //default/all

//if update
if ($id) {
  $category->load($id);
}

$general['name'] = "Cars";
$general['path'] = "1/3"; // 1/3 is root catalog
$general['description'] = "Great new cars";
$general['meta_title'] = "Cars"; //Page title
$general['meta_keywords'] = "car, automobile";
$general['meta_description'] = "Some description to be found by meta search robots.";
$general['landing_page'] = ""; //has to be created in advance, here comes id
$general['display_mode'] = "PRODUCTS_AND_PAGE"; //static block and the products are shown on the page
$general['is_active'] = 1;
$general['is_anchor'] = 0;
$general['url_key'] = "cars";//url to be used for this category's page by magento.


$category->addData($general);

try {
    $category->save();
    echo "Success! Id: ".$category->getId();
}
catch (Exception $e){
    echo $e->getMessage();
}