Encryption in WordPress

encrypt.php

<?php
/**
 * Protect direct access
 */

// This line is for WordPress
if ( ! defined( 'ABSPATH' ) ) die( 'Sorry cowboy! This is not your place' );

if( ! defined( 'SOME_RANDOM_STRING' ) ) define( 'SOME_RANDOM_STRING', 'ABHgtu^77y&6tgJy' );

if( ! class_exists( 'Helper_Encryption' ) )
{
    /**
     * Helper_Encryption
     */
    class Helper_Encryption
    {
        private $_key;
        
        private $_iv;
        
        static private $_instance;
        
        protected function __construct()
        {
            $this->_key = pack( 'H', SOME_RANDOM_STRING );
    
            $this->_iv = mcrypt_create_iv(
                mcrypt_get_iv_size( MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC ),
                MCRYPT_DEV_URANDOM
            );
        }
        
        public static function get_instance () {
            if ( ! isset( self::$_instance ) ) self::$_instance = new self();
            return self::$_instance;
        }
        
        public function encode( $string )
        {
            return base64_encode(
                $this->_iv .
                mcrypt_encrypt(
                    MCRYPT_RIJNDAEL_256,
                    hash( 'sha256', $this->_key, true ),
                    $string,
                    MCRYPT_MODE_CBC,
                    $this->_iv
                )
            );
        }
        
        public function decode( $encrypted )
        {
            $data = base64_decode( $encrypted );
            $iv = substr( $data, 0, mcrypt_get_iv_size( MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC ) );
        
            return rtrim(
                mcrypt_decrypt(
                    MCRYPT_RIJNDAEL_256,
                    hash( 'sha256', $this->_key, true ),
                    substr( $data, mcrypt_get_iv_size( MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC ) ),
                    MCRYPT_MODE_CBC,
                    $iv
                ),
                "\0"
            );
        }
        
    }
}

/**
 * Usage:
 * 
 * $e =  Helper_Encryption::get_instance();
 * $str = 'Hello Mars!';
 * $t = $e->encode( $str );
 * $e->decode( $t );
 * 
 */