Class mysqli not found error after upgrading to php 7.3

The problem seems to be related to the fact that after PHP 7.2 or PHP 7.3 is installed, the version of PHP 7.0 still continues to be used. And in spite of the fact that extensions, for example such as php7.0-mysql, do not disappear from the system, but they cease to be picked up by the PHP interpreter and the server can no longer fully work with PHP code.

The simplest way that will solve this problem is disabling PHP 7.0 for Apache and enabling PHP 7.2 for the web server, since in any case PHP 7.3 is installed automatically. To do this, execute:

 sudo a2dismod php7.0
 sudo a2dismod php7.2
 sudo a2enmod php7.3
 sudo a2disconf php7.0-fpm
 sudo a2disconf php7.2-fpm
 sudo systemctl restart apache2

Feel free to comment if you face same issue and this helps to resolve your problem or if you are not able to solve the problem.