How do I run multiple websites on the same server with the same code base in PHP?

[JASE17]

I have created a custom, specialized CMS with a number of clients on it, each with their own domain name, website, admin area and database, but all residing on the same server. Whenever I get a new client, I simply copy all of the code over, change 3 lines in a config file, and then all of their styles/data is taken out of the database or from uploads they post to the server from their own admin area.Sounds great so far right? Well when I recently decided to update all of the clients sites, this became a major pain. Obviously i had to change the code on each install. For a major update this is fine, but for frequent tweaks or the like, duplicating the effort of uploading becomes very annoying....I hope to someday have dozens or hundreds of clients, so the code will eventually have to be centralized so that changing it in one place updates it everywhere...how does one do that?