See these pages for more details on how to install PHP/phpMyAdmin:
https://help.ubuntu.com/lts/serverguide/php.html
https://help.ubuntu.com/lts/serverguide/phpmyadmin.html

$ sudo apt install php

Install required tools:

$ sudo apt install libapache2-mod-php php-mysql

Verify the version of PHP:

$ php -version

PHP 7.2.24-0ubuntu0.18.04.1 (cli) (built: Oct 28 2019 12:07:07) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.2.0, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.2.24-0ubuntu0.18.04.1, Copyright (c) 1999-2018, by Zend Technologies

$ sudo apt install phpmyadmin

Choose the web server that should be automatically configured to run phpMyAdmin.
Web server to reconfigure automatically: apache2

Configure database for phpmyadmin with dbconfig-common? yes

MySQL application password for phpmyadmin: phpmyadmin

Verify the version of phpMyAdmin:

$ dpkg -l | grep phpmyadmin

ii  phpmyadmin  4:4.6.6-5           all  MySQL web administration tool

Access the default page of phpMyAdmin: http://localhost/phpmyadmin

You need to setup a password for the 'admin' user:

$ sudo htpasswd /etc/phpmyadmin/htpasswd.setup admin

New password:
Re-type new password:
Updating password for user admin

Access the phpMyAdmin setup: http://localhost/phpmyadmin/setup



You might get this error: "phpMyAdmin - Error. Configuration already exists, setup is disabled!"
Check if the file "/etc/phpmyadmin/config.inc.php" already exists in the root phpMyAdmin directory.
Move/rename this file to fix this issue:

$ sudo mv /etc/phpmyadmin/config.inc.php /etc/phpmyadmin/config.inc.php.bak

Modify the phpMyAdmin alias:

$ sudo nano /etc/phpmyadmin/apache.conf

Alias /phpmyadmin1 /usr/share/phpmyadmin

Now you can access the phpMyAdmin using the URI: http://localhost/phpmyadmin1