Recently I have subscribed to Digital Ocean managed database and I was facing this issue where I was not able to connect to the database. I got the following error.

Error: Connection failed: The server requested authentication method unknown to the client

The reason was that Nginx configuration was using php 7.0 even though php -v command shows the output as 7.2

My droplet config:
Ubuntu 16.04
PHP 7.2
MySQL 5.7

So to change the default php version, I am using Nginx so I ran the following command.

update-alternatives --set php /usr/bin/php7.2
service nginx restart

If you are using apache, you can use the following commands

a2enmod php7.2
service apache2 restart

Still, when I echo the output of phpinfo() it shows that my website was using php7.0

Next, I decided to check the Nginx config file for any issues. To my surprise it was set to use php7.0

I changed to Nginx config to the following.

...

location ~ \.php$ {
    include snippets/fastcgi-php.conf;
    fastcgi_pass unix:/run/php/php7.2-fpm.sock;
}

...

After that restarted the service and I was able to connect to the database.

sudo systemctl restart nginx

More information regarding this issue can be found here. https://www.php.net/manual/en/mysqli.requirements.php