Why Facebook’s HHVM Is Shifting from PHP 7 to Hack: The Backend Evolution

Because PHP 7 and PHP 5 have major compatibility problems, the Facebook HHVM team decided to develop HHVM using Hack.

Facebook’s HHVM is a high‑performance PHP execution engine that has announced it will gradually remove its dependence on the latest major PHP version—PHP 7—and focus on the Hack language, a branch of PHP.

The next long‑term support version, HHVM 3.24, is scheduled for early 2018 and may be the last release that includes PHP 5 support.

The HHVM team explains that supporting both PHP 7 and Hack leads to compromises for both; therefore they plan to detach from PHP entirely to fully exploit Hack’s advantages without the legacy issues of PHP’s design.

PHP 7 diverges significantly from PHP 5, breaking backward compatibility. The HHVM team intends to follow a similar path, not concentrating on PHP 7, but providing a clear roadmap that makes Hack an excellent language for web development, free from the constraints of its PHP origins.

In recent years Facebook has been running Hack on HHVM. The team notes that Hack has addressed many of the shortcomings of PHP 5 that PHP 7 either fixed or left unresolved.

By reducing reliance on PHP, the HHVM team hopes that HHVM and Hack together will deliver a better, higher‑performance experience for developers. Improvements designed specifically for Hack include:

Enhanced Hack arrays that make type checking easier.

Elimination of destructors and references.

Use of garbage collection to achieve measurable performance gains.

Hack is built on the PHP ecosystem, and Facebook plans to make HHVM compatible with major PHP tools such as Composer and PHPUnit.

The ultimate goal is to give Hack its own core framework ecosystem, including the Hack standard library, TypeAssert for converting untyped data to typed data, and an autoloader with support for type aliases and functions.

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