I have a network drive mapped to drive letter X:\\ going to an external hard drive with the path of \"\\\\X-Drive\\Public\\Data\".
I am using Zend Server with Apac
For network shares you should use UNC names:
"//server/share/dir/file.ext"
Source
If you use the IP or hostname it should work fine:
$isFolder = is_dir("\\\\NAS\\Main Disk");
var_dump($isFolder); //TRUE
$isFolder = is_dir("//NAS/Main Disk");
var_dump($isFolder); //TRUE
$isFolder = is_dir("N:/Main Disk");
var_dump($isFolder); //FALSE