> Not only that, the entire codebase is full of STDOUTs and STDERRs and STDINs

Yeah, that was going to be my follow-up comment. I'm not so keen on creating a hacked solution throughout the codebase to "fix" a deficiency in HHVM. Particularly because HHVM isn't a mainstream project yet and is mostly at the whims of what Facebook wants to do with it.

Instead, what about creating a `hhvm-compat` package which includes a custom logger and whatever tweaks you need to get WP-CLI working properly? It will be a good exercise too in determining the other ways we need to make WP-CLI extendable. You can use #963 to run WP-CLI's internal test suite against HHVM on Travis.

Once you've got it working reliably, we can re-open this conversation about supporting HHVM in the main project.