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

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

namespace HH\Lib\Math;

function min_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\min().