Authentication on 2 sites by link?

Question

PHP

I can't think of a better way to do authentication on subdomains.

There are two sites
The first is based on the webtrees engine svn.webtrees.net/demo-stable/login.php?url=index.php
The
second is based on Gallery 3
www.opensourcecms.com/demo/1/343/Gallery

There are also a large number of users who do not distinguish a login from a password and generally have difficulty navigating the Internet (elderly people).
The information on the site - a pedigree with photographs - security is completely uncritical, but it also has nothing to do in the public domain.

Open ID and social networks are not good, because. users do not have such accounts and will not be able to register there.
It seems to me that the best solution would be to provide some kind of password link (as in Picasa's web albums), when you click on it, the login occurs on 2 sites at once. But since I'm not a web developer, I'm not sure. Your help is required. Please tell me where and what words to google? Maybe there are ready-made solutions?

Answer 1

Option 1: If we host both sites, then authorization can be done on one site, well, and authorize on the second automatically (a couple of lines of PHP codes ... well, let's say a couple of dozen). When authorizing on the second site - a redirect to the first, and then along the knurled one.
Option 2: General authorization page. This is the same as in the first option, but the authorization looks purely visually neutral without belonging to the first or second site.
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