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

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

Returns the arithmetic mean of the numbers in the given container


namespace HH\Lib\Math;

function mean(
  Container<num> $numbers,
): ?float;

To find the sum, see Math\sum(). To find the maximum, see Math\max(). To find the minimum, see Math\min().