static variable called in function giving error undefined codeigniter php

Question

I have defined static variable in controller but when I use that variable in functions it is giving undefined variable error.

Controller

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Quiz extends Admin_Controller {

    private static $secure_key = "aXXXXXXXXc";

    public function __construct()
    {
        parent::__construct();
    }

    public function edit($id)
    {
        try
        {
            $token = JWT::encode($postdata, $secure_key);
            echo "<pre>";print_r($token);exit; 
        }
        catch(Exception $e){
            $this->data['error'] = $e->getMessage();
            redirect('/','refresh');
       }
    }
}

$token gets printed properly with jwt but I am getting an error

Undefined variable: secure_key

I tried different methods to define $secure_key as

public static $secure_key = "aXXXXXXXc;
static $secure_key = "aXXXXXXXc;

I tried to define $secure_key in constructor also as $secure_key = "aXXXXXXXc;

but no use. Why so? Please help. I am using codeigniter 3

Answer 1

Recommended Method (Based on Security)

Define variables in config.php and access it. This will work like Global Variable

$config['secure_key'] = 'myKey';
$this->config->item('secure_key'); # get
$this->config->set_item('secure_key', 'NewKey'); # set

---

Access it like this

$this->$secure_key

As per Comment by cd001

self::$secure_key 

If function

$this->function_name();