It is important to shake things from time to time. But would be difficult to do it forever (considering growing user demographics). If a "shake up" results in any positive outcome one can still end up having only a temporary effect. In order to prolong this effect, we probably want to get to the situation where the following is sufficient:
Best form of moderation is self-moderation
I am a newbie here and I apologize for somewhat rhetorical tone. Nevertheless I hope I am making a useful point here.
What I would discuss further probably can be described in single sentence as user self-moderation and self-education by reading more meta SO:
To answer the OP's question I started by sorting php tag by votes and looked through the well rated answers on well rated questions, meaning that an analysis similar to OP's with an overall good SO style and quality in mind can be applied there as well.
One still does find a lot of duplicates/reposts (e.g. this). Occasionally users give comments "Why this answer gets only X upvotes while almost a similar answer got Y (where Y >> X)?". It seems obvious that acting and thinking like this people do not prevent post duplication in a natural manner. Why? Is it because they don't care? Somehow I doubt that most of the users stay indifferent to SO concepts while still actively making posts (at least not entire SO population). Next (arguable) guess would be that maybe users do not pay too much attention to the importance of following a good Q/A style because they have no idea about it :) ('said as a true captain')
Unfortunately, I don't think I am proposing an ultimate solution to the problem of a particular tag (php), but I would really like to share one simple (probably rather obvious) thought:
The more people read/participate in meta stackoverflow, the more they think and learn about better practises of a good Q/A style. They become part of a community in yet another, additional way by facing problems from the conceptual or designing perspective.
Not sure how representative my case is, but I must confess that the more time I spend on reading meta SO, the more I am getting into the problems discussed here - the more I care or get involved. In the past I tended to play a role of just a "passive" guest, a visitor that comes by to get a desired piece of information. Not sure why, but it did take me a while to start paying greater attention to the meta part of SO.
I just want to say that even reading meta does makes a difference. And maybe one of the ways to improve overall writing quality is indeed to attract more people to meta in many possible ways.
By no means do I think that what I suggest here is something extraordinary or novel. I just merely confirm that it seems to be working, at least in my case.
From what I can tell, you guys already did a great job by making meta SO more accessible to the visitor:
- There is a link to meta in the main menu (in the top navigation bar, in the bottom links, in the list of all of SE sites)
- SO FAQ and community wiki pages are linked to meta
- Certain questions get migrated to meta (which redirect users).
- ...(something I don't know/can't think of right now)
SO site seems to do a lot to help to transit from a visitor to a registered user, and from occasional contributor to a standards-aware author. Doing so is an obvious motivation of improving and educating people. I guess a constantly pending question would be how to do it even better?
Finally, It could be not at all a bad idea to consider different types of writing Q/A standards to be tag specific, custom enough (maybe including a possibility of extra obligatory input fields) to address writing quality of a particular tag subgroup.
Best analogy I can provide is a bug reporting: better structured reporting form optimizes testing cycle (requiring a screenshot, reproducibility instructions, software and environment versions, etc.).