Creating a global variable for all PHP pages?

I want to define a variable that I can reference on ALL of my PHP pages. Ideally, I'd like to do it in php.ini or some similar place so that I can modify it as I place code on different systems, but not need to make changes in lots of different places. I looked at the Configuration section of the manual, did web searches, tried getenv() and using the OS environment (we're mixed Windows and UNIX boxes, using Apache and mysql and PHP 4.1.1), and had zero luck. I was hoping that getenv() or _ENV[] would have worked. I tried creating something in php.ini by simply adding a line, with no luck. Is there some way of doing this? Warrick