I've found something I don't understand - hope someone can enlighten me!
I've been moving a static site over to WP and wanted to use a PHP include to replace the old HTML version. So I created the PHP file and put it in my old includes folder (in the root). But in the WP template I kept getting the path wrong.
I was playing around with <?php include('wp-includes/file.php'); ?> and discovered that this works -- even though file.php is not in that directory but in my old 'includes' directory.
I'm happy that it works -- but how did WP find that file? Should I move it to the 'wp-includes' directory? But if I did, would it get overwritten if I do an automatic upgrade in the future?