There have been other proposals for this, but please allow me to present this in a new, simplified approach.
Premise
We can mainly distinguish 4 types of tags on SO:
- [concept] (such as [memory-management], [cpu], or [binary-trees])
- [technology] (such as [php], or [mysql])
- [technology]-[feature] | [technology][feature] (such as [mysql-connect], or [html-attributes])
- [technology]-[version] | [technology][version] (such as [html5], or [c#3.0]
With the first 3 items it's pretty clear what this does.
The problem is [technology][version]. Tagging nomenclature becomes vague, askers will not know what to use (php, php5, or both?). Experts in these tags will register to several RSS feeds if they want to follow them, etc. People looking for answers will not find them due to mislabeling, and will post duplicate questions.
Proposal
To allow a tag to be either a parent or a child (not both) of another tag, and thus to have the means to organise [technology][version] tags as children of their respective [technology] parent.
In that way, tagging a question with either parent or child (say [php] or [php5], or both) will attract the attention of the proper experts. When looking up [php5], only the questions with the [php5] tag would show. When looking up [php], all its children would be included ([php4], [php5], etc.)
I propose nothing else. I'm not touching features or concepts, only versioning.
Because of the 2 level restriction, looking up tags would simply imply a modified query, adding a single OR statement:
...
WHERE tag_id = %tagSelected%
OR parenttag_id = %tagSelected%
In this way, if the tag is already a parent, the OR statement will return all children. If the tag is a child, the OR statement will return nothing.
Closing argument
In this way, we can re-assemble several categories into meaningful ensembles again, and stop much confusion, without having to retrace so many questions to retag them. Searching will give fuller results, and tagging will be more effective.
Supplemental argument: Parent tags can be sponsored for a higher price. $$$!!!