I agree with you that the general level of questions under the php tag is pretty poor. I spend most of my time on that tag and a lot of the questions are very basic. However, I do not agree that creating a wiki will help.
I think part of the problem is that the low barrier to entry for PHP makes it a popular language for beginners and amateurs. I used to fall into both camps, but now I hope its only the latter.
Nobody posting on SO for the first (second or third) time is going to read the tag wiki or any community wiki, they don't even read the How To Ask page when faced with it for their first question. Its only when you get interested in being a part of the SO community that you read that stuff.
My strategy has been to try and point out when people are trying to do something that is poor practice. If I answer such a question I will try and point out why doing what they are doing is not a good idea and present a better option. I have been down voted for it occasionally, but that's OK.
I also down vote answers that encourage poor practice and up vote those that encourage good practice.
Remember that (most) users don't post poor questions intentionally. Look at it as an opportunity to teach them how to do something properly and point them at resources that encourage good programming practices.
If we all do the same, maybe we can raise the standard of questions under the phptag.
I notice the zend-framework tag seems to be going the same way lately, so maybe my optimism is misplaced.