The Symphony 2.3 release opens up a whole new range of possibilities for developers to create awesome sites and applications. We've cleaned up a lot of the codebase and we are always looking at ways to make it leaner and more efficient. Because of this reason, we've decided that the Symphony 2.3 line will be the final releases that we test against PHP5.2.
We feel that with the recent release of PHP 5.4, now is a good time to end our relationship and look forward to the new possibilities that developing with a PHP 5.3+ codebase brings.