<?php
function randHash($len=32){    return substr(md5(openssl_random_pseudo_bytes(20)),-$len);}
 $rand = randHash();

class Payment_Model extends CI_Model {   
 function insertPayment($uid, $amount,$rand) {        
$this->db->insert('payments', array(          
 'amount' => (float)$amount,          
 'user' => $uid,            
 'hash' =>$rand,          
 'time' => time(),            
 'status' => 0      
 ));        
return $this->db->insert_id();    } ?>

$order_id = $this->Payment_Model->insertPayment($this->user['id'], $this->input->post('amount'));