OpenCart 🇺🇦

$this->language->load('product/compare');

$json = array();

if (!isset($this->session->data['compare'])) {
	$this->session->data['compare'] = array();
}

if (isset($this->request->post['product_id'])) {
	$product_id = $this->request->post['product_id'];
} else {
	$product_id = 0;
}

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

$product_info = $this->model_catalog_product->getProduct($product_id);

if ($product_info) {
	if (!in_array($this->request->post['product_id'], $this->session->data['compare'])) {	
		if (count($this->session->data['compare']) >= 4) {
			array_shift($this->session->data['compare']);
		}

		$this->session->data['compare'][] = $this->request->post['product_id'];
	}

	$json['success'] = sprintf($this->language->get('text_success'), $this->url->link('product/product', 'product_id=' . $this->request->post['product_id']), $product_info['name'], $this->url->link('product/compare'));				

	$json['total'] = sprintf($this->language->get('text_compare'), (isset($this->session->data['compare']) ? count($this->session->data['compare']) : 0));
}	

$this->response->setOutput(json_encode($json));