If you read this blog post closely, you will notice several interesting points in the Future Changes, which hint at making Hack a significantly saner language than PHP, especially around type-soundness.
> Several behaviors will be removed from PHP arrays; the general principle is that PHP arrays should work as “vec or dict”.
> Int-like string keys will not be converted to integers.
> Binding references to array elements will not be permitted.
> The values null, false, or uninitialized variables will not be able to be treated as arrays.
> Empty strings will not be able to be treated as arrays.
> Arrays will not be able to be compared to non-arrays.
> Non-arraykey values will not be able to be used as array keys and will not be automatically coerced.
> Arrays will not be able to be used with the plus (+) operator.
This is a massive change that goes a long way towards making the most prominent, core data structure of any php codebase be sound.
Other examples of introducing type soundness:
> Undefined constants will no longer be converted to strings
> ints will wrap in future releases, instead of being converted to floats, allowing the type of int + int to be int, instead of int + int = num.