Non centra con php, comunque si chiama virtualhost name-based.
In httpd.conf:

NameVirtualHost *:80

<VirtualHost *:80>
ServerName www.sito1.it
DocumentRoot /var/www/html/sito1
</VirtualHost>

<VirtualHost *:80>
ServerName www.sito2.com
DocumentRoot /var/www/html/sito2
</VirtualHost>


http://httpd.apache.org/docs/2.0/vhosts/name-based.html