HHVM 4.0.0

This release adds support for files, non-experimental support for HSL regular expressions, and removes several PHP behaviors. While this release does not ban and similar tags, various changes make HHVM unable to execute most PHP projects. PHP-incompatible changes in this release mostly fit in to one of three goals:

We are considering removing support for in the next release: the primary benefit would be a clearer error message (such as “HHVM no longer supports PHP code”) rather than more-specific messages such as parse errors which may be less useful.

Source: hhvm.com