It is much easier to do a gradual migration. Start with one part of your application, and bit by bit migrate your logic and application. The traditional way of doing such migrations is to create a new project and have parallel development on the old and the new version of the application. The problem with this, though, is that when you make a change to your old application, you have to make the same change in the new codebase, essentially doubling the amount of work for each feature you need to implement. [...] You could wrap your old application into your Symfony2 application, and have different parts of your application be handled by different versions of your codebase.

With the help of a bundle he created, IngewikkeldWrapperBundle that handles the rerouting of your requests based on where the requested resource exists (in the Symfony 1 or 2 codebase).

The transition was relatively painless. [...] What's nice about PHP5 is that it caught some errors that have been lingering in our code: PHP5 no longer allows a function to be defined twice, and some basic variable referencing errors that we missed previously.

They made the move to PHP5 for the latest versions of their applications, but have still stuck with the legacy, PHP4 versions for the time being to give customers a buffer period to make the move themselves. He also mentions changes to the way they make Ajax calls. John is a lead developer for both the ADOdb and PHPLens projects.