<?php
function siteURL()
{
return "http://example.com/dir/";
}
define('SITE_URL', siteURL()); //returns http://example.com/dir/
define('CKEDITOR', SITE_URL.'plugins/ckeditor/ckeditor.js');
// ignore the previous code is just to emulate your constants and functions
// the answer is the following line:
echo str_replace(SITE_URL,'',CKEDITOR);
?>