I love the concept of migrations found in Ruby on Rails. It allows you to use an agile methodology when developing web applications. You do not have to make a the full database design first before starting to develop your web application, you can start with a partial database design, make a working application then gradually evolve it in an easy way by adding more and more functionality till it reaches the full functionality you need.
The key here is that database migrations in Ruby on Rails make very easy to go back and forth between more or less functionality in the system you are building. I really hope that PHP would have some similar easy way that supports migrations. That way going truly agile with PHP would be possible as is the case with Ruby on Rail.