solution “Could not reliably determine the server’s fully qualified domain name, using … for ServerName”

To fix that problem, you need to edit the httpd.conf file.
Open the terminal and type this command:

$ sudo gedit /etc/apache2/httpd.conf

httpd.conf file will be blank. Now, simply add the following line to the file.

ServerName localhost

Save the file and exit from gEdit.
Finally restart the server.

$ sudo /etc/init.d/apache2 restart