Many Thanks for the reply, it seems you've spotted in the right direction.
(Also well spotted with the post on Stackoverflow, it is indeed mine!)

So, in a shell as root,
   > php -r 'print_r(get_loaded_extensions());' | grep mysql
Produces:
   > PHP Warning:  Module 'mysqli' already loaded in Unknown on line 0
   > [21] => mysqli

Whilst the same instruction interpreted via the browser does not show any 'Mysqli'.

That springs mutliple questions, then:
1) the warning in the output of the instruction in the shell does seem to indicate mutliple instances trying to load that module, correct ?
  However, I've looked for other 'php.ini' in the entire system (jail), but there's none. Where else could the instruction come from ?

2) In the php file to be interpreted by the browser (so with 'www' user), I've left the 'phpinfo()' and added the 'get_loaded_extensions()' instruction to be shown underneath. From 'phpinfo()', I can see:
   In table "Module Authors", module "Mysqli" and its authors.
I had assumed that this meant the module was loaded. Yet, the 'get_loaded_extensions()' on that same page does not list 'mysqli' (it does list 40 other modules).So what does the fact that Mysqli is shown in phpinfo() actually tells us ? That it is simply installed but not necessarily loaded ?

In 'php.ini', if I uncomment the line 'extension mysqli', I get the output above. If I leave it commented, I don't get the warning anymore, yet still the same output when exectuted as root in shell ([27] => mysqli).
The php webpage with the phpinfo() and get_loaded_extensions() instructions display the same things (and yes I had restarted Apache after making the change in php.ini).

So it does seem that there is an issue in the config if read by root Vs by 'www', and php.ini seems to not be the leading config file for PHP as the Mysqli module appears loaded (for root) regardless of having the corresponding line commented or not in php.ini.
I fear I would need another assistance to make a step further.

Thanks in advance !
Tanguy

Thanks again, but no luck.
I've searched for any other php.ini in the whole system, there is just one.

I've just looked in /var/log/httpd-error.log and can see:
AH01071: Got error 'PHP message: PHP Fatal error:  PHP extension "mysqli" is not loaded\n in /usr/local/www/apache24/data/photo/piwigo/include/functions_html.inc.php on line 388\n'

I think I'll have to bin the jail and start all over again, to see what happens....

Tanguy