My main concern is the trickle-down effect a major low-level engine addition causes. Your patch is just the tip of the iceberg which will cause dozens of people weeks of work to account for the new code all across the PHP ecosystem. The most complicated being the opcode cache support which really only can be written by a handful of people due to the complexity involved. Combine that with the fact that other projects who currently use annotations, perhaps not to the level of Doctrine, but still, state that they would have a hard time switching to this new approach it becomes really hard to commit all these people and all this time to this.

We are severely resource-constrained when it comes to people who can write solid low-level C code and we have to be very careful what we ask our volunteers to spend their time on. A volunteer developer who isn't excited about a feature is going to drag her feet and it will sit solidly at the bottom of the priority list for months, if not years. If a key piece of the eco-system isn't updated because of this one addition, it means that potential PHP 5.4 users may have to wait 6, 12, 18 months before they can migrate to the new version.

Therefore, low-level engine changes, syntax additions, or entirely new grammars as is the case here, face an uphill battle. If there is a way to currently solve the problem without major changes, even if it is an 80% solution that will weigh heavily against accepting the new code.

Without broad support and enthusiasm, especially from the people who have historically been the ones that write the code and track down and fix the bugs, low-level features like this are doomed, no matter how well-intentioned they are.