MariaDB 10 has been updated several times since the original post (currently on version 10.3.21) and there have been a number of minor changes which affect the instructions above.
Install MariaB10 and phpMyAdmin - as above.
Open MariaDB 10 first.
You will be forced to set a password. New security policy requires all passwords to be at least 14 characters long and contain mixed-case letters, numbers and special characters; it does not allow common passwords, or usernames repeated as passwords. This is the root password that you will use in setting up phpMyAdmin.
Close MariaDB 10 and open it again. Make sure the Enable TCP/IP connection box is ticked (see post by @aicarmic above).
Close MariaDB10 and open phpMyAdmin. Set it up as described in the original post, using user root and the MariaDB 10 password.
Don’t create a new database yet.
On the home screen of phpMyAdmin, create the new user first by clicking on the User Accounts tab at the top. Click on Add User Account about halfway down the page. Fill in the login details and tick the checkbox Create database with same name and grant all privileges
When you click GO the account and the database will be created.
Assuming that the user/database you created was called home_assistant, your configuration.yaml entry should now look something like this:
Well worth doing if you have a Synology NAS - speeds up the system and extends the life of your SD card.