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

This function is available as Math\max_by() 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_by<T>(
  Traversable<T> $traversable,
  (function(T):num) $num_func,
): ?T;

The value for comparison is determined by the given function. In the case of duplicate numeric keys, later values overwrite previous ones.

For numeric elements, see Math\max().