mywebsite.com work but www.mywebsite.com don't work well !! opencart

My Website (opencart) works great on mywebsite.com but i got problems when trying www.mywebsite.com, eg: icons on header and products disappear i tried to add a line on config.php:

<?php
// HTTP
define('HTTP_SERVER','http://mywebsite.com/');


// HTTPS
define('HTTPS_SERVER', 'http://ift.tt/1BhM8QL');


// DIR
define('DIR_APPLICATION', '/home/XXXX/public_html/catalog/');
define('DIR_SYSTEM', '/home/XXXX/public_html/system/');
define('DIR_DATABASE', '/home/XXXX/public_html/system/database/');
define('DIR_LANGUAGE', '/home/XXXX/public_html/language/');
define('DIR_TEMPLATE', '/home/XXXX/public_html/view/template/');
define('DIR_CONFIG', '/home/XXXX/public_html/system/config/');
define('DIR_IMAGE', '/home/XXXX/public_html/image/');
define('DIR_CACHE', '/home/XXXX/public_html/system/cache/');
define('DIR_DOWNLOAD', '/home/XXXX/public_html/download/');
define('DIR_LOGS', '/home/XXXX/public_html/system/logs/');

// DB
define('DB_DRIVER', 'mysql');
define('DB_HOSTNAME', 'localhost');
define('DB_USERNAME', 'XXXX_com1');
define('DB_PASSWORD', 'XXXXXXXXX');
define('DB_DATABASE', 'XXXX_com1');
define('DB_PREFIX', 'oc_');?>

define('HTTP_SERVER','http://ift.tt/tv1JJF');

under

define('HTTP_SERVER','http://mywebsite.com/');

nothing happened i'm beginner on opencart ,how can i solve this ?