Met dank aan deze website op https://linuxconfig.org, een werkende oplossing om het MySQL root wachtwoord weer werkend te krijgen op phpMyAdmin.

Instructions

Reset by using mysql_secure_installation

The simplest approach to reset MySQL database root password is to execute mysql_secure_installation program and when prompted entering your new root MySQL password:

$ sudo mysql_secure_installation
….
Please set the password for root here.
New password:
Re-enter new password:

Reset by using skip-grant-tables

If from some reason the above method fails follow the step below to use --skip-grant-tables to reset MySQL root password.

Let’s start by stopping the currently running MySQL database:

$ sudo service mysql stop

Next, create a /var/run/mysqld directory to be used by MySQL process to store and access socket file:

$ sudo mkdir -p /var/run/mysqld
$ sudo chown mysql:mysql /var/run/mysqld

Once ready manually start MySQL with the following linux command and options:

$ sudo /usr/sbin/mysqld –skip-grant-tables –skip-networking &
$ [1] 2708

Confirm that the process is running as expected:

$ jobs
[1]+ Running sudo /usr/sbin/mysqld –skip-grant-tables
–skip-networking &

At this stage we are able to access MySQL database without password:

$ mysql -u root
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.20-1ubuntu1 (Ubuntu)

Copyright (c) 2000, 2017, Oracle and/or its affiliates.
All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type ‘help;’ or ‘\h’ for help. Type ‘\c’ to clear the
current input statement.

mysql>

Using the MySQL session first flush privileges:

mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.00 sec)

Next, reset root password. The following commands will reset MySQL root password to 
linuxconfig.org: (= het nieuwe wachtwoord)

mysql> USE mysql;
Database changed
mysql> UPDATE user SET authentication_string=PASSWORD (“linuxconfig.org”) WHERE User=’root’;
Query OK, 0 rows affected, 1 warning (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 1
mysql> UPDATE user SET plugin=”mysql_native_password”
WHERE User=’root’;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 0

Quit MySQL session:

mysql> quit
Bye

Gracefully terminate current mysqld process:

$ sudo pkill mysqld
linuxconfig@ubuntu:~$ jobs
[1]+ Done sudo /usr/sbin/mysqld –skip-grant-tables –skip-networking

Lastly, start MYSQL database:

$ sudo service mysql start

If all went well you should now be able to login to your MySQL database with a root password:

$ mysql -u root –password=linuxconfig.org
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.7.20-1ubuntu1 (Ubuntu)
Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type ‘help;’ or ‘\h’ for help. Type ‘\c’ to clear the current input statement.
mysql>

Opm: Het kan zijn dat phpMyAdmin opnieuw moet worden geïnstalleerd, tijdens deze installatie wordt gevraagd of de bestaande databases verwijderd moeten worden, pas daar dus op als je dat niet wil.
Eerst sudo apt-get remove phpmyadmin
Daarna
sudo apt-get install phpmyadmin php-mbstring php-gettext -y