Why can mysql CLI connect, but not WordPress?

I just had this problem on Debian 9.1 using MariaDB, but this should be the same in Oracle MySQL. I could log in easily from the command line, but PHP could not access the database.

It took me a few minutes to figure out, but I remembered from when I used to use Arch Linux on an old server (a dumb idea by the way) that the command mysql_secure_installation was needed to get it to accept connections.

Try it out, this worked for me.

NOTE: RUNNING ALL PARTS OF THIS SCRIPT IS RECOMMENDED FOR ALL MariaDB      SERVERS IN PRODUCTION USE!  PLEASE READ EACH STEP CAREFULLY!In order to log into MariaDB to secure it, we'll need the currentpassword for the root user.  If you've just installed MariaDB, andyou haven't set the root password yet, the password will be blank,so you should just press enter here.Enter current password for root (enter for none):OK, successfully used password, moving on...Setting the root password ensures that nobody can log into the MariaDBroot user without the proper authorisation.Set root password? [Y/n] n ... skipping.By default, a MariaDB installation has an anonymous user, allowing anyoneto log into MariaDB without having to have a user account created forthem.  This is intended only for testing, and to make the installationgo a bit smoother.  You should remove them before moving into aproduction environment.Remove anonymous users? [Y/n] y ... Success!Normally, root should only be allowed to connect from 'localhost'.  Thisensures that someone cannot guess at the root password from the network.Disallow root login remotely? [Y/n] y ... Success!By default, MariaDB comes with a database named 'test' that anyone canaccess.  This is also intended only for testing, and should be removedbefore moving into a production environment.Remove test database and access to it? [Y/n] y - Dropping test database... ... Success! - Removing privileges on test database... ... Success!Reloading the privilege tables will ensure that all changes made so farwill take effect immediately.Reload privilege tables now? [Y/n] y ... Success!Cleaning up...All done!  If you've completed all of the above steps, your MariaDBinstallation should now be secure.Thanks for using MariaDB!