In order to make the transistion for users upgrading to new minor (middle number increment) and major (leading number increment) version of PHP easier, the PHP project has recently begun adding a README with upgrade information with new releases. This guide steps through any issues that users are likely to be faced when upgrading their PHP installations.
Unfortunately often it is not so trivial to see the entire scope of a given change. As a result sometimes unwanted BC breaks end up in releases or the information about unexpected but unavoidable changes is lacking. This is especially the case for long time behaviors that are however never intended to be relied upon. This is why it is imparative that end users test the release candidates of new PHP releases and report back any issues or changes that are noticed. This way the change can either be reverted, adapted to be more reasonable or at the very least be documented.
You can report issues via the bug tracker, the internals mailing list, IRC or even here. This is particularily immediatly relevant for the upcoming 5.2.0 release that is already at RC2 and the bundled README.UPDATE_5_2.