Magento 1.7 – Directory Separator Returning Wrong Separator

I'm trying to get a function working to re size any image using a custom module and helper class. For the record I'm running this on a local XAMPP server.

If I pass a image url and a destination url using '/' all works fine. However I need to implement this with DS for future proofing. The following is an example line of code I'm using. with the DS instead of '/'

$newPath = Mage::getBaseDir(Mage_Core_Model_Store::URL_TYPE_MEDIA) . DS . "resized" . DS . $fileName;

This returns (without the http or localhost at the start because stackoverflow doesn't allow that even in code) "/test/media/resized\catalog\category\testcat.jpg"

How do I make it so that DS correctly returns forward slashes?