Loose Coupling
Ryan Norris has written up a good article on the benefits of tiered application development in PHP. As he correctly points out, there is a difference between keeping your logic and presentation completely separate and dividing it along the more appropriate line of business logic vs. presentation and its related logic. I have long been an advocate of this latter approach and a lot of it is reflected in Smarty, at least when I used to work on it. If you developing a PHP application consisting of more than a couple of pages, it would probably be a good idea to read this article and apply it to your project.
