php.ini is located at "/etc" (along with three other files named php.ini.applesaved, php.ini.default, and php.ini.server-default, by the way).
Also, by the way, I found this location from the command line (Terminal) by typing "cd .." until I was at the root of "Server HD", then used "ls" to work my way back down and find the file. It seems the folder "/etc" is really an alias to "/private/etc" which is where the php.ini file lives.
Although the Finder normally won't show these invisible folders and files, and Spotlight doesn't find them, I've since discovered that if you know where the folder is then you can use Finder's "Go to Folder..." menu item to see it.
So... I used TextWrangler to add the line include_path = "/Library/WebServer/Documents/" to php.ini.
I found that cURL was already enabled.
The version of PHP installed with Snow Leopard Server is 5.3.1.
Finally, now the test page loads successfully.
One remaining possible problem is that I haven't addressed this statement:
"If you are accessing databases that contain dates and times, install the pear date package."
Hopefully, as with the cURL thing, it's a non-issue. I really don't want to have to figure out what the heck it takes to do this. And, I do need to work with dates. So, we'll see...
Hope this helps another server newbie some day.