Virtual Hosts
Apache2 has the concept of sites, which are separate configuration files that Apache2 will read. These are available in /etc/apache2/sites-available. By default, there is one site available called default this is what you will see when you browse to http://localhost or http://127.0.0.1. You can have many different site configurations available, and activate only those that you need.

As an example, we want the default site to be /home/user/public_html/. To do this, we must create a new site and then enable it in Apache2.

To create a new site:

Copy the default website as a starting point.

sudo cp /etc/apache2/sites-available/default /etc/apache2/sites-available/mysite

Edit the new configuration file in a text editor “sudo nano” on the command line or “gksudo gedit”, for example:

gksudo gedit /etc/apache2/sites-available/mysite

Change the DocumentRoot to point to the new location. For example, /home/user/public_html/
Change the Directory directive, replace to 
You can also set separate logs for each site. To do this, change the ErrorLog and CustomLog directives. This is optional, but handy if you have many sites
Save the file
Now, we must deactivate the old site, and activate our new one. Ubuntu provides two small utilities that take care of this: a2ensite (apache2enable site) and a2dissite (apache2disable site).

sudo a2dissite default && sudo a2ensite mysite

Finally, we restart Apache2:

sudo /etc/init .d /apache2 restart

If you have not created /home/user/public_html/, you will receive an warning message

To test the new site, create a file in /home/user/public_html/:

echo '<b>Hello! It is working!</b>' > /home/user/public_html/index .html

Finally, browse to http://localhost/

The above content was taken from the website at https://help.ubuntu.com/community/ApacheMySQLPHP