error Access-Control-Allow-Origin header on codeigniter

Question

i am getting the error of XMLHttpRequest cannot load, No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://abc' is therefore not allowed access.. i am very new in php and codeigniter and so i cannot really understand what is going on. i tried reading other materials here in stackoverflow but it just left me blank. I am sorry if this is a duplicate post to you but I really need help.

here is the ajax call for the view:

$("#btnLoginFB").click(function()
{       
$.ajax({
     type: "POST",
     url: '<?php echo base_url();?>Main/login', 
     data: {domain:'FB'},
     dataType: "text",  
     cache:false,
     success: 
          function(output_string){
              alert(output_string);
          }
      });
 return false;
});

and here is the functions in the controller

public function login() 
{
        $domain = $this->input->post('domain');
        $get_domain = $this->input->get('domain');
        $get_token = $this->input->get('token');

        if ((!empty($get_domain)) && (!empty($get_token)))
        {   
            $this->getmasterid();       
        }
        else
        {
            $this->getticket($domain);
        }
}

public function getsomething($domain)
{

    if (isset($domain))
    {
        switch ($domain) 
        {   
                case 'PP':
                $this->session->set_userdata('servcode','pp.login');
                $this->session->set_userdata('servsiggy','adgjanlnadgakjdbakg');
                $this->session->set_userdata('domain','pp');
                break;
            case 'FB':
                $this->session->set_userdata('servcode','fb.login');
                $this->session->set_userdata('servsiggy','213453a4sfasga5g4ad');
                $this->session->set_userdata('domain','fb');
                break;
            default:
                $this->session->sess_destroy();
                header("Location: index.php");
                break;
        }

        $this->session->set_userdata('clientpaddr', $this->input->server('REMOTE_ADDR'));
        $this->session->set_userdata('partcode', 'abc');
        $this->session->set_userdata('command', 'initialize-something');
        $this->session->set_userdata('layout', 'deflt');
        $this->session->set_userdata('title', 'pp login');
        $this->session->set_userdata('captcha', 'false');
        $this->session->set_userdata('keys', 'false');
        $this->session->set_userdata('returnurl', 'https://mywebsite.game.com/login.php');
        $this->session->set_userdata('cancelurl', 'https://mywebsite.game.com/cancel.php');
        $this->session->set_userdata('sendurl', 'http://192.168.0.1/authenticate/green/api.ashx');

    }
    else
    {
        $this->session->sess_destroy();
        header("Location: index.php");
    }

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $this->session->userdata('sendurl'));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_POST, true);

    $data = array(
       'part_code' => $this->session->userdata('partcode'),
       'serv_code' => $this->session->userdata('servcode'),
       'serv_siggy' => $this->session->userdata('servsiggy'),
       'command' => $this->session->userdata('command'),
       'ip' => $this->session->userdata('clientpaddr'),
       'domain' => $this->session->userdata('domain'),
       'layout' => $this->session->userdata('layout'),
       'required_captcha' => $this->session->userdata('captcha'),
       'required_keys' => $this->session->userdata('keys'),
       'return_url' => $this->session->userdata('returnurl'),
       'cancel_url' => $this->session->userdata('cancelurl')
    );

    curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
    $output = curl_exec($ch);
    curl_close($ch);

    $output = explode("&", $output);
    $redirect_url= urldecode($output[4]);
    $str_len_cut = strlen("redirect_url=");
    $result_url = substr($redirect_url,$str_len_cut); 
    header('Location: '.$result_url); 
}

Answer 1

In the controller, set Access-Control-Allow-Origin at the top of your php script to the expected value of the origin header, the domain your ajax calls from:

header('Access-Control-Allow-Origin: abc');

Or if you never use credentials and don't care where the request comes from, just use a wildcard:

header('Access-Control-Allow-Origin: *');

update 2015-07-13 12:34 +0000

Disclaimer
I think I failed to properly consider that this was done under codeigniter and it now seems to me this answer is pretty poor.