I was going to post this in the PHP forum but then I realized the topic wasn't specifc to PHP.

Whenever I design an application from ground up my approach is usually to build the framework starting from the same point a person using my application would.

I start with the installer, then user registration, login/logout, main content, and then any additional features. This often means that from time to time I must go back and make changes to the installer or other features I built in earlier. In spite of this, however, it works for me, even though when I think about it it would make more sense to build the bulk of my application first and then just quickly take it apart and make an installer.

I'm curious as to what approach everyone else takes.

I usually use a driver script and add pages controlled by the driver. The driver usually uses a core class to do a lot of the work. The advantage to this is that updating the driver or driver core updates all pages.

I always write the installer last.

Same here on installer code. Last thing to be done.

I always break the application up into small components.
This makes each piece easy to test. Also, small parts tend to be much simpler thus, less likely to have bugs. Then my "application" code is little more than glue to hold them all together.
Another bonus, often components can be reused in other applications (though not as much as I'd like)
Like Chris, the installer is the last thing.