Hello,

Finally, I managed to upgrade from 1.4 to 1.5 (I had a library problem on my localhost server, and I moved to php7.1)

I have added the Zenpage extension and modified index.php to display pages.
I would like to add search results from Zenpage articles to my basic theme.

You explain here : https://www.zenphoto.org/news/theming-tutorial/#--part-4-theming-zenpage

Updates to standard theme pages required
Adding search results for Zenpage items on search.php: Adding search results to a search page is actually a little more complicated and may look a little confusing at first. Search results for Zenpage news articles and pages are not paginated and only shown on the first search results page. Since Zenpage is an optional plugin we need to make sure that we set Zenpage results to 0 if the plugin is not enabled to avoid errors.

I have had a look to examples, but it's not clear what code I need in my search.php file to succeed.

Take a look at the Zenpage theme's search.php. That theme was generally based on the basic theme back then. There is also a bare bones example here: https://github.com/zenphoto/demo-theme/blob/master/demo_theme-zenpage-cms-extras-files/search.php

Thank you acrylian,
problem solved :
search.php is not enough : it calls
and the corresponding file is functions .php.

So, I copied search.php AND functions.php from the zenpage theme, inside my basic theme folder, and added my template.

Now, I can search everywhere.

Yes, indeed `` is needed if you want this foldout behaviour. It is by no means really required. I personally don't like it anymore and it will surely not return in any future theme planned ;-)