This API is part of the Hack Standard Library, not HHVM itself.
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This function is available as C\every() in Facebook's www repository.

Returns true if the given predicate returns true for every element of the given Traversable


namespace HH\Lib\C;

function every<T>(
  Traversable<T> $traversable,
  ?(function(T):bool) $predicate = null,
): bool;

If no predicate is provided, it defaults to casting the element to bool.