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...