i thought about overloading an existing config option to avoid adding another one (like maybe SMTP_SERVER set to false), but that seemed a bit ugly and error prone.  php's object system isn't as clean as other languages, so you'd have to remember to check with === false as needed, and make sure to not expand without checking first.

you might be able to argue that any meaningful code should be gated anyways at a higher level (like DIGEST_ENABLED does in this patch), but that seems like it'd be error prone.