Get Wordpress Child Theme Path in Wordpress

I have a parent and child theme. In my child theme I declared a variable CHILD_DIR so I can add custom JS and CSS files to the my child theme's folder structure. I did this in my functions.php file in my child theme by doing this: ``` define( 'CHILD_DIR', get_stylesheet_directory_uri() ); ``` This produces a URL which is what I want since I'm using it like this: ``` wp_enqueue_script('custom', CHILD_DIR .'/custom.js', array( 'jquery' )); ``` I also added some additional folders to my child theme which I want to use within a Wordpress page. I tried to do something like this: ``` require_once CHILD_DIR . '/Zend/Loader.php'; ``` I get an error since it's trying to provide a URL and I want a menu path to include the PHP and not try and make it an HTTP request. How can I define a path for the PHP instead of a URL to default to my child theme?