Everything is described here: 
 
https://forums.opensuse.org/showthread.php/576937-mysqli-doesnt-work?highlight=mysqli

however in short:

Once having done a distribution upgrade from leap 15.3 to 15.4 the PHP MySQLi
module won't load when running from the web unless you add the line:

extension=mysqlnd

somewhere in a used php.ini.

However, when running PHP from the command line (CLI mode) the module loads
correctly which also means that when you add the line then from then on in CLI
mode you will get the warning:

PHP Warning:  Module 'mysqlnd' already loaded in Unknown on line 0

I guess you could avoid the warning by adding the line in:
/etc/php7/apache/php.ini

which I should add the fact into the above referenced forum post.