CI Class for create Captcha Image

Captcha.php

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

class Captcha extends CI_Controller {

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

//Settings: You can customize the captcha here
    public function index($rand = 100) {
        $image_width = 100;
        $image_height = 30;
        $characters_on_image = 5;
        $font = 'monofont.ttf';

//The characters that can be used in the CAPTCHA code.
//avoid confusing characters (l 1 and i for example)
        $possible_letters = '23456789AbCdDFfGhJkLiMmNPQqRrSTtVwWxYzZ';
        $random_dots = 25;
        $random_lines = 5;
        $captcha_text_color = "0x142864";
        $captcha_noice_color = "0x142864";

        $code = '';
        $i = 0;
        while ($i < $characters_on_image) {
            $code .= substr($possible_letters, mt_rand(0, strlen($possible_letters) - 1), 1);
            $i++;
        }

        $font_size = $image_height * 0.90;
        $image = @imagecreate($image_width, $image_height);

        /* setting the background, text and noise colours here */
        $background_color = imagecolorallocate($image, 255, 255, 255);

        $arr_text_color = $this->hexrgb($captcha_text_color);
        $text_color = imagecolorallocate($image, $arr_text_color['red'], $arr_text_color['green'], $arr_text_color['blue']);

        $arr_noice_color = $this->hexrgb($captcha_noice_color);
        $image_noise_color = imagecolorallocate($image, $arr_noice_color['red'], $arr_noice_color['green'], $arr_noice_color['blue']);

        /* generating the dots randomly in background */
        for ($i = 0; $i < $random_dots; $i++) {
            imagefilledellipse($image, mt_rand(0, $image_width), mt_rand(0, $image_height), 2, 3, $image_noise_color);
        }

        /* generating lines randomly in background of image */
        for ($i = 0; $i < $random_lines; $i++) {
            imageline($image, mt_rand(0, $image_width), mt_rand(0, $image_height), mt_rand(0, $image_width), mt_rand(0, $image_height), $image_noise_color);
        }

        /* create a text box and add 6 letters code in it */
        $textbox = imagettfbbox($font_size, 0, $font, $code);
        $x = ($image_width - $textbox[4]) / 2;
        $y = ($image_height - $textbox[5]) / 2;
        imagettftext($image, $font_size, 0, $x, $y, $text_color, $font, $code);

        /* Show captcha image in the page html page */
        header('Content-Type: image/jpeg'); // defining the image type to be shown in browser widow
        imagejpeg($image); //showing the image
        imagedestroy($image); //destroying the image instance
//        $_SESSION['letters_code'] = $code;
        $this->session->set_userdata('captcha_letters_code', $code);
    }

    private function hexrgb($hexstr) {
        $int = hexdec($hexstr);
        
        return array("red" => 0xFF & ($int >> 0x10),
            "green" => 0xFF & ($int >> 0x8),
            "blue" => 0xFF & $int);
    }
}