edit /etc/phpmyadmin/apache.conf

<IfModule mod_rewrite.c>
  <IfModule mod_ssl.c>
    <Location /phpmyadmin>
      RewriteEngine on
      RewriteCond %{HTTPS} off
      RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI}
    </Location>
  </IfModule>
</IfModule>