On the HHVM blog (Facebook's HipHop Virtual Machine project) they've posted about support for PHP 7 features that they've been working into HHVM and Hack to support backwards compatibility with this latest major version of the PHP language.
For those that haven’t been following along, the next version of the PHP language, version 7.0.0, was very recently released. Those of us working on HHVM offer our congratulations to all the contributors to this latest release! We’re all really excited to see this release come out the door, and for what it means for the future of PHP.
The release has implications for HHVM as well. [...] The HHVM project is committed to continuing to support the evolving PHP language, and as such we are proud to announce that the current nightly releases have support for all major PHP 7 features, and the upcoming 3.11.0 stable release will be the first release of HHVM with support for the major PHP 7 features.
There are some backwards compatibility issues that PHP 7 introduced but the HHVM project has opted to support both the PHP 5 and 7 users simultaneously. This is made as low friction as possible either through silent compatibility or though a
hhvm.php7.all ini configuration setting.