Not quite an answer to your question but slack engineering wrote a paper [1] on migrating their code base from PHP proper to Hack.

Specifically they used [2] 'partial mode' since "this loosens several restrictions to ease migration"

[1] https://slack.engineering/hacklang-at-slack-a-better-php/

[2] https://docs.hhvm.com/hack/source-code-fundamentals/program-...

EDIT: This would indicate that Hack may perhaps support standard PHP syntax but not the other way around.

EDIT: According to this [3] they are very compatible.

[3] https://www.oreilly.com/library/view/hack-and-hhvm/978149192...

I wish more people would read 'Hack and HHVM', written by Owen Yamauchi, formerly a member of Facebook’s core Hack and HHVM teams.

http://shop.oreilly.com/product/0636920037194.do

The hidden lesson for me was that rewriting the code in is not the only option. Another option is to slowly improve the language/runtime itself until you've essentially switched it out underneath the application, which is what happened at Facebook. Meanwhile keep refactoring the code to take advantage. (Granted, this is isn't an option for a small company.)

I work at Facebook and sometimes write www code. When I was interviewing and thought about writing PHP code, I didn't get a warm and cozy feeling, being reminded of terrible PHP code I've seen (and written myself) in the 2000s as a "webdev". Thanks in part to the advances described in the book, the codebase is definitely not like that; it's easily the best large scale codebase I've ever seen (I've seen 3-4).

My thoughts in blog form (written 3 months after I joined):

http://bytepawn.com/hack-hhvm-second-system-effect.html