รบกวนขอสอบถามความเข้าใจเกี่ยวกับ PHP OOP หน่อยครับ พอดีกำลังเริ่มต้่นศึกษาจากตัวอย่าง Class ครับ

	public function insert() {
		$this->language->load('catalog/category');

		$this->document->setTitle($this->language->get('heading_title'));
		
		$this->load->model('catalog/category');
		
		if (($this->request->server['REQUEST_METHOD'] == 'POST') && $this->validateForm()) {
			$this->model_catalog_category->addCategory($this->request->post);

			$this->session->data['success'] = $this->language->get('text_success');

			$url = '';

			if (isset($this->request->get['page'])) {
				$url .= '&page=' . $this->request->get['page'];
			}
						
			$this->redirect($this->url->link('catalog/category', 'token=' . $this->session->data['token'] . $url, 'SSL')); 
		}

		$this->getForm();
	}

$this->load->model('catalog/category');

if (($this->request->server['REQUEST_METHOD'] == 'POST') && $this->validateForm()) {
			$this->model_catalog_category->addCategory($this->request->post);

			$this->session->data['success'] = $this->language->get('text_success');

			$url = '';

			if (isset($this->request->get['page'])) {
				$url .= '&page=' . $this->request->get['page'];
			}
						
			$this->redirect($this->url->link('catalog/category', 'token=' . $this->session->data['token'] . $url, 'SSL')); 
		}

$this->model_catalog_category->addCategory($this->request->post);

$this->session->data['success'] = $this->language->get('text_success');

			if (isset($this->request->get['page'])) {
				$url .= '&page=' . $this->request->get['page'];
			}

$this->redirect($this->url->link('catalog/category', 'token=' . $this->session->data['token'] . $url, 'SSL')); 

public function addCategory($data) {
		$this->db->query("INSERT INTO " . DB_PREFIX . "category SET parent_id = '" . (int)$data['parent_id'] . "', `top` = '" . (isset($data['top']) ? (int)$data['top'] : 0) . "', `column` = '" . (int)$data['column'] . "', sort_order = '" . (int)$data['sort_order'] . "', status = '" . (int)$data['status'] . "', date_modified = NOW(), date_added = NOW()");

		$category_id = $this->db->getLastId();
				
		if (isset($data['image'])) {
			$this->db->query("UPDATE " . DB_PREFIX . "category SET image = '" . $this->db->escape(html_entity_decode($data['image'], ENT_QUOTES, 'UTF-8')) . "' WHERE category_id = '" . (int)$category_id . "'");
		}
		
		foreach ($data['category_description'] as $language_id => $value) {
			$this->db->query("INSERT INTO " . DB_PREFIX . "category_description SET category_id = '" . (int)$category_id . "', language_id = '" . (int)$language_id . "', name = '" . $this->db->escape($value['name']) . "', meta_keyword = '" . $this->db->escape($value['meta_keyword']) . "', meta_description = '" . $this->db->escape($value['meta_description']) . "', description = '" . $this->db->escape($value['description']) . "'");
		}

		// MySQL Hierarchical Data Closure Table Pattern
		$level = 0;
		
		$query = $this->db->query("SELECT * FROM `" . DB_PREFIX . "category_path` WHERE category_id = '" . (int)$data['parent_id'] . "' ORDER BY `level` ASC");
		
		foreach ($query->rows as $result) {
			$this->db->query("INSERT INTO `" . DB_PREFIX . "category_path` SET `category_id` = '" . (int)$category_id . "', `path_id` = '" . (int)$result['path_id'] . "', `level` = '" . (int)$level . "'");
			
			$level++;
		}
		
		$this->db->query("INSERT INTO `" . DB_PREFIX . "category_path` SET `category_id` = '" . (int)$category_id . "', `path_id` = '" . (int)$category_id . "', `level` = '" . (int)$level . "'");

		if (isset($data['category_filter'])) {
			foreach ($data['category_filter'] as $filter_id) {
				$this->db->query("INSERT INTO " . DB_PREFIX . "category_filter SET category_id = '" . (int)$category_id . "', filter_id = '" . (int)$filter_id . "'");
			}
		}
				
		if (isset($data['category_store'])) {
			foreach ($data['category_store'] as $store_id) {
				$this->db->query("INSERT INTO " . DB_PREFIX . "category_to_store SET category_id = '" . (int)$category_id . "', store_id = '" . (int)$store_id . "'");
			}
		}
		
		// Set which layout to use with this category
		if (isset($data['category_layout'])) {
			foreach ($data['category_layout'] as $store_id => $layout) {
				if ($layout['layout_id']) {
					$this->db->query("INSERT INTO " . DB_PREFIX . "category_to_layout SET category_id = '" . (int)$category_id . "', store_id = '" . (int)$store_id . "', layout_id = '" . (int)$layout['layout_id'] . "'");
				}
			}
		}
						
		if ($data['keyword']) {
			$this->db->query("INSERT INTO " . DB_PREFIX . "url_alias SET query = 'category_id=" . (int)$category_id . "', keyword = '" . $this->db->escape($data['keyword']) . "'");
		}
		
		$this->cache->delete('category');
	}