There are a number of piwigo sites that are no longer functioning due to a problem with php and mysqli.
The error I got was
   Fatal error: Class 'mysqli' not found in (...)/include/dblayer/functions_mysqli.inc.php on line 67
I dug around for a long time without success, though I did find several non-functioning piwigo sites with the same error message. (google that error line to see)

My problem arose when I upgraded Debian and picked up php7, though not a fully configured one.
Not all of the php7 packages needed were picked up, and php5 was still on my system.
My fix (your mileage may vary) was to purge all of the php5 packages and install the missing php7 packages.
I did this using "dpkg -l | grep php5" to flag the packages to delete, and corresponding php7 packages to add on.
The corresponding php7 packages were named "php-blat" rather than "php5-blat".
I had to enable the apache2 php7 extension (both load and conf in mods-enabled).
I then restarted apache2 with a service call.
All is now well.

Hope this helps someone.