HHVM 4.15 is released! This release marks the end of support for 4.9; 4.10-4.14 remain supported, as do the LTS releases 3.30 and 4.8.

Highlights

IDE integrations now show docblocks for namespaced functions.
Fixed an issue that could lead to CLI server clients hanging on exit in some Linux environments.
Improved typing of noreturn methods.

Breaking Changes

Untyped variadics now raise type errors; replace with mixed ...$foo.
inout parameter types are now invariant - both co- and contra- variant. They were previously incorrectly treated as contravariant.
instanceof is now a parse error. In 4.14.x, it was not a parser error, but no longer refined types. In HHAST 4.14.x, hhast-migrate --instanceof-is can be used to replace expressions of the form $x instanceof Foo with $x is Foo, and expressions of the form $x instanceof $some_expression with \is_a($x, $some_expression).

In a future release, all support for calling functions by-reference will be removed. Typechecker enforcement can be enabled now with the disallow_byref_calls=true .hhconfig option.
Support for "${foo}" will be removed, and can be replaced with "{$foo}"; hhast-migrate --dollar-brace-variable-migration can be used to migrate code, and the disable_outside_dollar_str_interp=true .hhconfig setting can be used to enforce this immediately.