Codeigniter 'Message: Undefined index: site_lang' issue at first load - php

Question

I have developed a website and it is working fine in my localhost.
But I wanted to test it in a local area network before going live.
There is an issue I am facing which is at first load in a browser or any other machine it shows below error until I change the language once. When I change the language once then the issue is never happening again.
I am new and this is my first project so can anyone help me in this regard.
Thanks in advance
A PHP Error was encountered
Severity: Notice
Message: Undefined index: site_lang
Filename: models/companies.php
Line Number: 15
Backtrace:
File: C:\xampp\htdocs\fp\application\models\companies.php
Line: 15
Function: _error_handler
File: C:\xampp\htdocs\fp\application\controllers\home.php
Line: 19
Function: get_companies
File: C:\xampp\htdocs\fp\index.php
Line: 315
Function: require_once
Here is my Model:
//---- Table Companies ---
public function get_companies()
{
// Retrieve titles for all languages
$sql = "SELECT * FROM `ci_companies` WHERE `co_id` = 1";
// Retrieve appropriate title according to the chosen language in the system
$sql = "SELECT `co_id`, `co_name`, SUBSTRING(`co_detail`,1,100) AS `co_detail`, `co_img`, `co_img2` FROM `ci_companies` WHERE `co_lang` = '".$_SESSION['site_lang']."'";
$query = $this->db->query($sql);
return $query->result();
}
And here is my controller:
function switchLang($language = "") {
$language = ($language != "") ? $language : "english";
$this->session->set_userdata('site_lang', $language);
redirect($_SERVER['HTTP_REFERER']);
}

Answer 1

Try like this.error due to wrong use of single quotes.
$language = $this->session->userdata('site_lang');//or $_SESSION['site_lang']
$sql = "SELECT `co_id`, `co_name`, SUBSTRING(`co_detail`,1,100) AS `co_detail`, `co_img`, `co_img2` FROM `ci_companies` WHERE `co_lang` = '{$language}'";