Old versions of mysqlnd worked with legacy database passwords. I had to update the database to use more secure password format before the mysqlnd plugin would connect from WordPress 5.0.3 to php 7.3
PHP Warning: mysqli::__construct(): (HY000/2000): mysqlnd cannot
connect to MySQL 4.1+ using the old insecure authentication. Please use an
administration tool to reset your password with the command
SET PASSWORD = PASSWORD('your_existing_password'). This will store a new,
and more secure, hash value in mysql.user. If this user is used in other
scripts executed by PHP 5.2 or earlier you might need to remove the old-
passwords flag from your my.cnf file in /root/db-test.php on line 2