Throughout 2016, Slack began migrating from PHP5 to Hack. They cite several well-known challenges inherent to PHP, including surprise type conversions, inconsistency around reference semantics, inconsistencies in the standard library, and the fact that “PHP tries very, very hard to keep the request running, even if it has done something deeply strange.”

To overcome these challenges while maintaining the unique values of PHP, Slack turned to Hack, a gradual typing system for PHP. Hack runs on the HipHop Virtual Machine, or HHVM, an open source just-in-time (JIT) environment for PHP.

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Taking PHP Seriously – Several People Are Coding (slack.engineering)
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