Codeigniter: Encryption Dynamic URL

./application/config/autoload.php
$autoload['libraries'] = array('my_encryption');
./application/controllers/encryption.php
<?php if (!defined('BASEPATH')) exit('No direct script access allowed');

class Encryption extends CI_Controller {

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

     public function index() {
          $data['encode'] = $this->my_encryption->encode('my_dynamic_url');
          $data['decode'] = $this->my_encryption->decode('HoDTZBLPrchP3hG3XCmKNhSxQNOk4n8qOlKqAkgLdwM');

          $this->load->view('encryption', $data);
     }

}

/* End of file encryption.php */
/* Location: ./application/controllers/encryption.php */
./application/libraries/MY_Encryption.php
<?php if (!defined('BASEPATH')) exit('No direct script access allowed');

class MY_Encryption {

     var $skey = "KQYsG4Hi201ajyEzOSGzr4MVfw"; // you can change it

     public function safe_b64encode($string) {
          $data = base64_encode($string);
          $data = str_replace(array('+', '/', '='),array('-', '_', ''),$data);
          return $data;
     }

     public function safe_b64decode($string) {
          $data = str_replace(array('-', '_'),array('+', '/'),$string);
          $mod4 = strlen($data) % 4;
          if ($mod4) {
               $data .= substr('====', $mod4);
          }
          return base64_decode($data);
     }

     public function encode($value) {
          if (!$value) {
               return false;
          }
          $text = $value;
          $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
          $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
          $crypttext = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $this->skey, $text, MCRYPT_MODE_ECB, $iv);
          return trim($this->safe_b64encode($crypttext));
     }

     public function decode($value) {
          if (!$value) {
               return false;
          }
          $crypttext = $this->safe_b64decode($value);
          $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
          $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
          $decrypttext = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $this->skey, $crypttext, MCRYPT_MODE_ECB, $iv);
          return trim($decrypttext);
     }

}

/* End of file MY_Encryption.php */
/* Location: ./application/libraries/MY_Encryption.php */
./application/views/encryption.php
<?php echo $encode;?>
<br/>
<?php echo $decode;?>