Using $this when not in object context in Codeigniter

I know this question already has answers but I did not understand so asked again So please help.

What does this error actually mean ?

Fatal error: Using $this when not in object context in C:\wamp\www\provisioning\application\controllers\customer\provisioning.php on line 27

public function index(){
  $users = $this->data['tenant_users'] = $this->customer_user_m->tenant_users();
  $domain = $users[0]['domain'];
  $site = $users[0]['site_key'];
  $tenant_id = $users[0]['tenant_id'];
  $site = $this->session->userdata('site');
  $user_table = $this->session->userdata('user_table');

  function getOTAURLExt($ext){
    var_dump($this);
  }

  function getOTAURLSite(){
    echo "Site executed";
  }

  $this->db->select('*');
  $this->db->where('id', $tenant_id);
  $this->db->from('tenant');
  $query = $this->db->get();
  $result = $query->result_array();

  if(empty($this->input->post('md'))){
    $URL = getOTAURLSite($site);
  }else{

    $username = $result[0]['username'];

    $table_user = $username . "_users";


    $this->db->select('*');
    $this->db->where($table_user . '.site_key', $site);
    $this->db->join('mlcsites', 'mlcsites.site_key =' . $table_user . '.site_key');
    $this->db->from($table_user);
    $query_table = $this->db->get();
    $information = $query_table->result_array();


    $ext = $information[0]['ext'];


    $count = count($information);
    $found = false;
    for($i = 0; $i < $count; $i++){
      $domain = $information[$i]['domain'];
      $ext = $information[$i]['ext'];

      $hash = do_hash($ext . "@" . $domain, 'md5');

      if($hash == $this->input->post('md')){

        $found = true;
        break;
      }
    }

    if($found == true){
      $URL = getOTAURLExt($ext);
    }
  }
  if(empty($URL)){

  }

  $this->data['subview'] = 'provisioning/index';
  $this->load->view('_layout_main', $this->data);
}

What could be the possible solution to this?