Advice sought on PHP site maintenance

Hi, I manage several sites for my company. Some are running our own service to about 80 customers and others are running a service for some (5 and growing) of our customers. Its the latter one that I need advice on. I have an application where each customer has a website on our service. The functionality and layout are almost identical throughout these sites and I am striving to move any differences into configuration files. However, when I make a change, I then need to make that change on each site. I would like, if possible to maintain a single set of web pages and have that work for all sites. I currently use PEAR::DB and Smarty templating. The current url syntax is www.mysite.com/client/ and I would like to keep them thinking that they each have their own unique site. Can anyone suggest a structure for this? MTIA George in Oxford