Hi leatherback,
I'm also Dutch, but I'll write English because may be some other people want to read these posts.
I tried the following:
mkdir("Upload/".$_POST['NewDir'],0777);
The permissions will be 0755.
Also I tried the following:
chmod("Upload/".$_POST['NewDir'],0777);
But the permissions are still 0755.
$_POST['NewDir'] = the foldername.