This API is part of the Hack Standard Library, not HHVM itself.

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.

If you're looking for C\all, this is it.