using PhpMyAdmin version 4.5.4.1deb2ubuntu2, you can set the variables in /etc/phpmyadmin/config-db.php
so set $dbserver to your server name, e.g. $dbserver='mysql.example.com';
<?php
##
## database access settings in php format
## automatically generated from /etc/dbconfig-common/phpmyadmin.conf
## by /usr/sbin/dbconfig-generate-include
##
## by default this file is managed via ucf, so you shouldn't have to
## worry about manual changes being silently discarded. *however*,
## you'll probably also want to edit the configuration file mentioned
## above too.
##
$dbuser='phpmyadmin';
$dbpass='P@55w0rd';
$basepath='';
$dbname='phpmyadmin';
$dbserver='localhost';
$dbport='';
$dbtype='mysql';