get all categories in magento

private function getCategories() {
	$categories = Mage::getModel('catalog/category')
                    ->getCollection()
                    ->addAttributeToSelect('*')
                    ->addIsActiveFilter();
 
	$all = array();
 
	foreach ($categories as $c) {
	   $all[$c->getId()] = $c->getName();
	}
 
	return $all;
}