Apache

You need at least the following setup:

<virtualhost *:80>
  ServerName example.com
  ServerAlias example.com www.example.com
  ServerAdmin [email protected]
  DocumentRoot /var/www/html/example_com
  ErrorLog /var/log/httpd/example_com_error
  CustomLog /var/log/httpd/example_com_access common
  <Directory "/var/www/html/example_com">
    Options FollowSymLinks Indexes
    AllowOverride All
    Order Allow,Deny
    Allow from all
    DirectoryIndex index.php
   </Directory>
</VirtualHost>