I'm not a PHP-guru, but it's entirely possible that they're playing around with some background configurations that would make performance tweaks in some sectio

I'm not a PHP-guru, but it's entirely possible that they're playing around with some background configurations that would make performance tweaks in some sections of the code difficult if they're defined variables. I guess this is an implementation of a semi-defined variable. It should probably be noted that there may be a tiny bit of performance improvement when calling isset instead of defined when checking for variable set. I'm not sure if that's one of the considerations since it would definitely be a case of premature (and ultimately pointless) optimization anywhere else, but since it's Facebook, they may have gone that far to squeeze every bit out of it they can.