Why/where should I use a chmod?

[dynamot]

I am beginner level in php. (I come from a ColdFusion back ground). I am currently working on a bulletin board (vBulletin)

Say my website is http://xyz.com
and the forums is at http://xyz.com/forums/

On the webroot of my website, I want to check if a user is logged in. The file I am using to check is website\check.php
One of the files that I need is at website/forums/globals.php. Looks like I cannot do a simple include or require like \forums\global.php or forums\globals.php on a page in my webroot. A gentleman here suggested that I first need to do a chmod() to that directory and then change it back.

Here is the code I have tried and failed with error message:
<?php
// Turn off all error reporting
//error_reporting(0);
require_once('/forums/global.php');
?>

Also I tried this:
<?php
// Turn off all error reporting
//error_reporting(0);

require_once('forums/global.php');
?>

Both above script fail with the error message:
Warning: require_once(/forums/global.php) [function.require-once]: failed to open stream: No such file or directory in E:\staging\websitename\check.php on line 5
Fatal error: require_once() [function.require]: Failed opening required '/forums/global.php' (include_path='.;C:\php5\pear') in E:\staging\websitename\check.php on line 5

*********************
But using chmod helps.
<?php
// Turn off all error reporting
//error_reporting(0);

chdir('forums');
require_once('global.php');
?>

This concept is alien to me. Why cant I access the "globals.php" just by using include/require?

I have php5 running on IIS