I'm wondering if it's really necessary to put the full version number in each source file. By doing this, in each release each PHP source file changes (and so its checksum).

What would you think about only including the x.y version number (4.0, 4.1), and a SVN revision number (per file, with Subversion keyword substitution)? Then, for an upgrade to a newer maintenance release, from x.y.w to x.y.z, not all PHP source files need to be updated, only a smaller set of changed files between x.y.w and x.y.z can be used.