I'm using Ubuntu desktop 16.04 (upgraded from 15.10).

I had installed phpmyadmin from by apt-get install phpmyadmin. It works if I go to localhost/phpmyadmin but I cannot log into it as root.

I have searched a lot for it. I have found many sources in which they suggest to alter /etc/phpmyadmin/config.inc.php and replace the user and password with 'root' and '' (empty for password). But my config.inc.php is different from theirs. For example in my file, there is no line for user and password and it seems it gets it automatically from another file which is /etc/phpmyadmin/config-db.php. Despite this, I have changed the user and password in that file, but now I get this error:

#1698 - Access denied for user 'root'@'localhost'

What I should do?

Phpmyadmin Version: 4.5.4.1deb2ubuntu1
mysql Ver 14.14 Distrib 5.7.12, for Linux (x86_64) using EditLine wrapper

MySQL 5.7 changed the secure model: now MySQL root login requires a sudo.

I.e., phpMyAdmin will be not able to use root credentials.

The simplest (and safest) solution will be create a new user and grant required privileges.

1. Connect to mysql

sudo mysql --user=root mysql

2. Create a user for phpMyAdmin

Run the following commands (replacing some_pass by the desired password):

CREATE USER 'phpmyadmin'@'localhost' IDENTIFIED BY 'some_pass';
GRANT ALL PRIVILEGES ON *.* TO 'phpmyadmin'@'localhost' WITH GRANT OPTION;
FLUSH PRIVILEGES;

If your phpMyAdmin is connecting to localhost, this should be enough.

3. Optional: allow remote connections

Remember: allow a remote user to have all privileges is a security concern.

With this in mind, if you want this user to have the same privileges during remote connections, additionally run (replacing some_pass by the password used in Step #2):

CREATE USER 'phpmyadmin'@'%' IDENTIFIED BY 'some_pass';
GRANT ALL PRIVILEGES ON *.* TO 'phpmyadmin'@'%' WITH GRANT OPTION;
FLUSH PRIVILEGES;

4. Update phpMyAdmin

Using sudo, edit /etc/dbconfig-common/phpmyadmin.conf file updating user/password values in the following sections (replacing some_pass by the password used in Step #2):

# dbc_dbuser: database user
# the name of the user who we will use to connect to the database.
dbc_dbuser='phpmyadmin'

# dbc_dbpass: database user password
# the password to use with the above username when connecting
# to a database, if one is required
dbc_dbpass='some_pass'

I was facing the same problem when using mariaDB with phpmyadmin (Ubuntu 16.04LTS).

Prerequisites:

1) Install MariaDB

sudo apt-get -y install mariadb-server mariadb-client
sudo mysql_secure_installation (answer to some interactive questions):
 Enter current password for root (enter for none): <enter>
 Set root password: n
 Remove anonymous users: n
 Disallow root login remotely: n
 Remove test database and access to it: n
 Reload privilege tables now: Y

If you want to uninstall mariaDB:

sudo apt-get purge mysql-server mysql-client mysql-common mysql-server-core-5.5 mysql-client-core-5.5
sudo rm -rf /etc/mysql /var/lib/mysql
sudo apt-get autoremove
sudo apt-get autoclean

2) Install phpmyadmin

sudo apt-get -y install phpmyadmin (and answer some interactive questions)
 Configuring phpmyadmin:
 Web server to reconfigure automatically: apache2
 Configure database for phpmyadmin with dbconfig-common: Yes
 MySQL application password for phpmyadmin: <blank>

3) In apache2, create a symbolic link to phpmyadmin

sudo ln -s /etc/phpmyadmin/apache.conf /etc/apache2/conf-available/phpmyadmin.conf
sudo a2enconf phpmyadmin.conf
sudo service apache2 restart

Ok, now, if you follow Rael's instructions, you'll be able to login into phpmyadmin, but, at least for me, I wasn't able to create new databases, as a red message came up: No privileges (or some similar message)

The fix was by reconfiguring phpmyadmin, and by answering some interactive questions.

sudo dpkg-reconfigure phpmyadmin
<Ok>
Reinstall database for phpmyadmin: <Yes>
Connection method for MySQL database server for phpmyadmin: TCP/IP
Host name of the MySQL database server for phpmyadmin: localhost
Port number for the MySQL service: 3306
MySQL database name for phpmyadmin: phpmyadmin
<Ok>
MySQL username for phpmyadmin: root
MySQL application password for phpmyadmin: pass # OR ANY PASSWORD YOU WANT
Password confirmation: pass
Name of the database's administrative user: root
Web server to reconfigure automatically: apache2
An error occurred while installing the database: ignore

Now if you try connect into phpmyadmin (localhost/phpmyadmin) using

username: root
password: pass

you'll be able to create databases.

In Ubuntu 16.04 for Mysql 5.7 check below table information and perform necessary settings:

mysql> SELECT User, Host, plugin FROM mysql.user;
+------------------+-----------+-----------------------+
| User | Host | plugin |
+------------------+-----------+-----------------------+
| root | localhost | auth_socket |
| mysql.session | localhost | mysql_native_password |
| mysql.sys | localhost | mysql_native_password |
| debian-sys-maint | localhost | mysql_native_password |
| phpmyadmin | localhost | mysql_native_password |
| root | % | mysql_native_password |
+------------------+-----------+-----------------------+

Check if root is having auth_socket plugin then run below command:

mysql> UPDATE user SET plugin='mysql_native_password' WHERE User='root';
mysql> FLUSH PRIVILEGES;
mysql> exit;

For completeness, I've found a solution to my trouble in using MariadB version 10.1.23. The syntax to use in order to setup a new user is similar to one reported into the post above from @Rael Gugelmin Cunha, I put here my solution for reference to others facing same trouble:

root@raspberrypi:# sudo mysql --user=root mysql
use mysql;
CREATE USER 'phpmyadmin'@'%' IDENTIFIED BY 'some_password';
GRANT ALL PRIVILEGES ON *.* TO 'phpmyadmin'@'%' WITH GRANT OPTION;
FLUSH PRIVILEGES;

Regards