PHP define is not working sometimes on some servers

Question

I am working on a PHP web app with another developer. We have 3 total different environments:

Configuration 1: Windows XAMPP, PHP 5.3.5
Configuration 2: Turnkey LAMP, PHP 5.3.X (turnkey-lamp-11.2-lucid-x86) - not sure which ver, running from virtualbox on a mac
Configuration 3: production server, hostgator shared running CentOS, php 5.3.9 fastcgi

Configuration 2 sometimes will not recognize the defines that are handled in our settings.php file which is require_once at the top of any page which uses it. The defines work fine on Configuration 1 and 3 always. There was nothing of interest in the apache logs, other than the errors generated as a result of the define being treated as a string.

I've searched for a while and cannot find anything that describes this issue. Configuration 2 is the other developer, and i do not have access to his machine right now. Looking for suggestions on how to track this issue down. I think we are going to try a new instance of the VM, using an upgraded turnkey-lamp, however i'd really like to root cause this.