I upgraded just a Debian server from Stretch to Buster. Now I notice there are both PHP 7.0 and PHP 7.3 installed. Is it supposed to be this way? I have not found documentation about PHP upgrades. I have mediawiki websites on that hosts, nono of them work now. Static websites and ikiwiki work OK.
Yes, PHP 7.0 is not deleted when upgrading. Have you changed the paths set for PHP? (or have they been changed without you knowing) Any errors in the logs of those webs?
For closure: I removed all PHP 7.0 packages (with apt purge php7.0-common), did a2enmod php7.3, restarted apache and PHP started working.