Hack is a statically typed programming language developed by Facebook (now Meta) that originally evolved from PHP. It runs on the HipHop Virtual Machine (HHVM), a custom runtime built by Facebook as a high-performance replacement for the standard PHP interpreter. Hack and Tyhp both add static typing to PHP-like syntax, but they take fundamentally different approaches.
Hack was created at Facebook in 2014 to address the type safety and performance issues the company faced while maintaining one of the world's largest PHP codebases. Hack adds a comprehensive type system to PHP syntax, including generics, nullable types, type aliases, async/await, and a powerful type checker. It runs on HHVM rather than the standard PHP runtime (php-fpm/php-cli), and over the years has diverged significantly from PHP, removing certain PHP features and adding entirely new constructs.
The most significant difference is the runtime. Hack requires HHVM, a separate virtual machine that replaces the PHP runtime entirely. Tyhp compiles to standard, readable PHP code that runs on any PHP 8.2+ runtime. This means Tyhp code works with your existing PHP infrastructure — your web server, hosting, extensions, debugging tools, and deployment pipeline all remain unchanged.
Hack has diverged substantially from PHP over the years. HHVM dropped support for running plain PHP code in 2018 (version 3.30), and Hack has since introduced syntax and semantics that are incompatible with PHP. Tyhp is a typed superset of the PHP language: it compiles to standard PHP that runs on a PHP runtime. .php files in a Tyhp project are passed through; <?tyhp source requires types and restricts some PHP features (see Lost and Changed Functionality).
Because Tyhp outputs standard PHP, it integrates seamlessly with the PHP ecosystem — Composer packages, PHP extensions, PHP frameworks (Laravel, Symfony, etc.), and existing PHP tools all work without modification. Hack's HHVM ecosystem is separate and significantly smaller, with many PHP packages requiring modification or re-implementation to work on HHVM.
Hack has a large engineering team at Meta and introduces many new operators, control structures, and language constructs beyond what PHP offers. Tyhp is more conservative: it starts from PHP syntax and adds constructs where they are worth the extra surface. Some PHP features are restricted in <?tyhp (for example eval() is off by default, and undeclared properties are allowed only on \stdClass) so the compiler can type-check; see Lost and Changed Functionality. Tyhp does not replace the PHP runtime.
Hack has many strengths. Its type system is mature and battle-tested at Facebook's scale. If you are willing to adopt HHVM as your runtime and your project does not depend heavily on the standard PHP ecosystem, Hack offers a powerful and well-supported language. Tyhp is a different solution: a typed superset that compiles to PHP so you can stay on a PHP runtime and the PHP ecosystem.
