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

This function is available as Math\max() in Facebook's www repository.

Returns the largest element of the given Traversable, or null if the Traversable is empty


namespace HH\Lib\Math;

function max<T as num>(
  Traversable<T> $numbers,
): ?T;

For a known number of inputs, see Math\maxva(). To find the smallest number, see Math\min().