coopster - 10:27 pm on Aug 20, 2013 (gmt 0)
PHP handles the mish-mash of mixed slashes on Windows internally, if I remember correctly.
Plesk is a nightmare. You probably need to add an Apache configuration for your domain in order to access the directory you need if it is not in the DOCUMENT ROOT or a subdirectory thereof. Even if it is in your include path you may still need to add it to the conf:
php_admin_value open_basedir "C:\PathToSiteRoot\dmdocuments"