// 加密数据并写到cookie里
$cookie_data = $this -> encrypt("nowamagic", $data);
				
$cookie = array(
	'name'   => '$data',
	'value'  => $cookie_data,
	'expire' => $user_expire,
	'domain' => '',
	'path'   => '/',
	'prefix' => ''
);
$this->input->set_cookie($cookie);

// 加密
public function encrypt($key, $plain_text) {   
	$plain_text = trim($plain_text);   
	$iv = substr(md5($key), 0,mcrypt_get_iv_size (MCRYPT_CAST_256,MCRYPT_MODE_CFB));   
	$c_t = mcrypt_cfb (MCRYPT_CAST_256, $key, $plain_text, MCRYPT_ENCRYPT, $iv);   
	return trim(chop(base64_encode($c_t)));   
}   

使用的时候再解密：

if( isset($_COOKIE['data']) )
{
	//用cookie给session赋值
	$_SESSION['data'] = decrypt("nowamagic", $_COOKIE['data']);
}

function decrypt($key, $c_t) {   
	$c_t = trim(chop(base64_decode($c_t)));   
	$iv = substr(md5($key), 0,mcrypt_get_iv_size (MCRYPT_CAST_256,MCRYPT_MODE_CFB));   
	$p_t = mcrypt_cfb (MCRYPT_CAST_256, $key, $c_t, MCRYPT_DECRYPT, $iv);   
	return trim(chop($p_t));   
}