This is an annotation from Stig Bakken:
The mode on your directory is affected by your current umask. It will end
up having (<mkdir-mode> and (not <umask>)). If you want to create one
that is publicly readable, do something like this:
<?php
$oldumask = umask(0);
mkdir('mydir', 0777); // or even 01777 so you get the sticky bit set
umask($oldumask);
?>