Recently, when I was restarting apache2 server then I encountered following problem:

**"AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message"**

Solution for the given problem is below two lines:

1.echo "ServerName localhost" | sudo tee /etc/apache2/conf-available/fqdn.conf
2.sudo ln -s /etc/apache2/conf-available/fqdn.conf /etc/apache2/conf-enabled/fqdn.conf 

For more visit this site: https://help.ubuntu.com/community/ApacheMySQLPHP
Enjoy !