There are (were) two problems.
First one is that there is a bug in the version of mythweb that is in the debian repository. I retrieved the latest mythweb from subversion, and the php error went away.
Second problem is my interpretation of the install document.
I went thru it three times from scratch and each time received the same results.
After entering "http://<local ip address>" on the browser address line, this is displayed:
This is the default web page for this server.
The web server software is running but no content has been added, yet.
If I enter "http://<local ip address>/mythweb", I get: Oops! This link appears to be broken." what I really get is the mythweb index listing
I believe that means I'm missing something in one of the config files -
but I know next to nothing about apache and network/server configuration.