I recently upgraded PHP5.X to PHP7.3 on my Pi 3 and also moved to Raspbian Buster from Jessie.
Now I get this message when I try to open the port 80 where I could earlier see my Wordpress site.
Your PHP installation appears to be missing the MySQL extension which is required by WordPress.
I use Apache and MariaDB and these are working. I installed
Php7.3-mysql as well.
PHP is working correctly. I created a test.php file with this content
<?php echo “hello world”; ?> and it works on the browser.