> Per the UPGRADING document found in the PHP 7 tarball, mhash was
> removed

That's not quite right. Actually, ext/mhash has already been
integrated into ext/hash as of PHP 5.3.0, but it still had it's
own module entry, so one could check for its presence with

  extension_loaded('mhash')

As of PHP 7.0.0 only this module entry has been removed, so one
has to use something like

  function_exists('mhash')

to check whether the mhash functionality is available. Other than
that there have been no changes (i.e. all mhash functions are
still available if PHP is configured with --with-mhash).

Since the PHP manual is currently supposed to document both PHP 5
and 7 we cannot easily move the respective documentation to the
hash extension. It seems to me that it would be sufficient to
document the special status of this "extensions" on its book's
frontpage.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.