Explore the journey of migrating Slack's multi-million line PHP codebase to Facebook's Hack programming language in this keynote address from the International PHP Conference 2019. Delve into the surprising, challenging, and beneficial aspects of the migration process, examining the pros and cons of Hack and identifying areas where the PHP community can draw inspiration. Learn about key topics such as the Language Server Protocol, type errors, generics, static type checking, inference, and the Hack Standard Library. Discover practical insights on fixing bugs, automated tooling, database interactions, and async/await functionality. Gain valuable lessons on test pyramids, unit tests, and best practices for considering a switch to Hack. Understand the intricacies of this incrementally typed adventure and its implications for large-scale PHP projects.