Решил проблему так:
header.php
$this->load->model('catalog/p_review');
$p_review_total = $this->model_sale_order->getTotalReviews('');
$data['p_review_total'] = $p_review_total;
$data['review'] = $this->url->link('catalog/p_review', 'user_token=' . $this->session->data['user_token'] . '&status=0', true);
model/p_review.php
public function getTotalReviews($status) {
$query = $this->db->query("SELECT COUNT(*) AS total FROM " . DB_PREFIX . "p_review WHERE status = '" . $status . "'");
return $query->row['total'];
}