Hacking the regitry is really ugly ;-) We should not advice people to do so.
If it's inside PHP, it should be documented, otherwise it's mystical undocumented feature. We should advice to not use it of course.
I mean: we should mention that it is possible to set it through the registry (it is documented in the search order with the key name), so advanced users can make use of it. But otherwise we should provide instructions to set the PHPRC var, and not to tweak the registry. In fact I would object to any documentation on *how* to tweak the registry in the manual, except if there is no other way. It is clearly positive to document *where* to tweak the registry, but not the *how*. Those, who are advanced users, know the *how*, others should not be instructed towards this way.
Goba
