IMO (as someone with 45k rep on StackOverflow, largely in PHP) - the best way is to modify the "flag" option on answers - and have an extra option called "out of date". If someone flags an answer as out of date, then anyone with a gold badge in that tag (i.e. PHP) - can review and accept or decline the flag.

If 5 gold people accept it - then the answer is formally highlighted as out of date, or even deleted.

You could raise this on Meta Stackoverflow as one possible way to start: https://meta.stackoverflow.com/

edit: There probably should be another flag option - called "security" - where even if an answer "works" and is "in date" - people can flag it as insecure due to a better option. Think of all the stupid SQL injection answers. You can downvote it to hell - but sometimes they should just be flagged/deleted.