Sesion already sent problem

[eluser]new developer[/eluser]
I have a problem saying session headers already sent. I worked the same code on Wamp but i installed Xampp recently. It gives me an error. My code is really simple as below. Can anyone please help me rectify this.

Code:

<?php

class Welcome extends Controller {

    function Welcome()
    {
        parent::Controller();    
    }
    

    function index(){
    $data['main'] = 'adminLogin';
      $data['title']= 'Welcome to my first aplication';
    $this->load->vars($data);
    $this->load->view(dashboard);
    }

      function verifyMe(){
      if($this->input->post('username')){
      $u = $this->input->post('username');
      $p = $this->input->post('password');
      $this-> MAdmins-> verifyUser($u,$p);
    if ($_SESSION['userid'] > 0){
    redirect('admin/dashboard','refresh');
    }
    }
         $this-> session-> set_flashdata(error  , 'Login failed');
       $this->load->view(adminLogin);
      }
}

?>