On the PHP 10.0 blog today Stas points out some packages from Zend that are the source versions of the Zend Server platform:

I was asked about PHP going with Zend Server, specifically from which sources it is built - as we don't ship source packages for the builds. Since Zend Server includes PHP build that can have some patches applied from SVN past the release (i.e. if the package has version 5.2.10 it might have some patches that were in SVN 5.2 branch past 5.2.10 tag) - I think it is important that people know what they are going to run if the use Zend Server.

The two packages, php-5.2-source-zend-server and php-5.3-source-zend-server, can be installed just like any other rpm/deb package and will give you full access to the source of the latest Zend Server installation. There's no Changelog included yet, but it should be "Coming Soon" according to Stas.