Пожалуйста, используйте ниже код:
Сначала Вы должны загрузить библиотеку сессий в: config-> autoload.php
$autoload['libraries'] = array('session'); // Load session Library in autoload.php
if ($_POST)
{
$sessionData = [
'p_id' => $this->input->post('pid'),
'p_name' => $this->input->post('pname'),
'p_qty' =>$this->input->post('viewproduct_details_qty'),
'p_price' => $this->input->post('pprice'),
'p_discount' => $this->input->post('pdiscount'),
'p_dprice' => $this->input->post('pdiscountprice'),
'p_imagename' => $this->input->post('pimagename'),
];
echo "<pre>";
print_r($sessionData);
$this->session->set_userdata('YOUR_SESSION_NAME',$sessionData); // You have to give session name
print_r($this->session->userdata('YOUR_SESSION_NAME')); // You Can Access Session by name
//$this->load->view("add_to_cart_page_session");
}