Surely with both the logic and presentation changing the best thing is to have them both seperate. As long as the interface is well defined then the two can be changed with minimal interaction

Agreement to the N^th power! Especially if the presentation maintainers and the logic maintainers are different groups of people.

With shifting requirements it's best to try and keep everything decoupled from everything else.

(Update: Not that this is a pro-Perl, anti-PHP statement. You can mangle the code and presentation together just as easily in Perl.)