PHP mkdir() and fopen() does not work - permissions problem? umask problem?

The following PHP script fails to create the directory. It will also fail to create the file (when the directory already exists).

ini_set('error_reporting', E_ALL);

define('ABSPATH', $_SERVER['DOCUMENT_ROOT']);
echo ABSPATH . '<br /><br />';

$dir_to_make = ABSPATH . '/aaatest';
$file_to_make = ABSPATH . '/aaatest/aaatest.txt';

echo umask() . '<br />';

mkdir($dir_to_make) or die('could not create directory');
fopen($file_to_make) or die('could not open/create file');

The umask() is returning a value of 18. The document root has a period in it ( /var/www/blah/websitename.com/httpdocs ).

I don't completely understand umask(), nor am I sure of how to properly use it. I don't know if that's the problem or not, but it does seem to be likely. Should I change the umask, create the file/directory, then change it back? What should the umask be to change/make/edit files/directories? Should the server be configured differently?