Assuming a standard wordpress installation using nginx server, here are the steps to keep wordpress running after upgrading Ubuntu.
1. Edit nginx config file:
sudo vim /etc/nginx/sites-available/{YOUR_SITE}
and modify the line:
fastcgi_pass unix:/run/php/php7.2-fpm.sock;
with correct version of php7.x-fpm.sock (should be one higher than the one in nginx). If you don’t know the version of php-fpm use this command to find out:
sudo apt list --installed | grep php
and look for php<version>-fpm.
2. Install php libraries which were removed as obsolete.
PHP_VERSION=7.2 sudo apt-get install php$PHP_VERSION-cli php$PHP_VERSION-cgi php$PHP_VERSION-fpm php$PHP_VERSION-mysql
PHP_VERSION should be the same in the one in nginx config file.
3. Restart php$PHP_VERSION-fpm:
sudo systemctl restart php7.2-fpm.service
4. Restart nginx:
sudo systemctl restart nginx.service