It's in the original download file for the 3.8.4 Patch 2 and also 3.8.3 version of the global.php file. I didn't look if it was in older versions or not.
I did toy around with that earlier today and made it
And tried
Code:
chdir('./forums/');
but neither seemed to work. I was able to get to admincp but when I'd log in, it just kept me in a loop.
The below is/was the default value originally.