There are very few questions that eventually could use php, which should be used when there isn't a more specific tag. The tag excerpt says the tag should not be used (for example) in the following questions:
- Questions about how to use a Drupal function
- Questions about the hook to use
- Questions about how to implement a hook
- Questions about the documentation of a Drupal function
The problem is that, on Stack Exchange sites, when a tag is used on the majority of the questions, it stops to being useful. That is the reason why the tag excerpt for modules says not to use it to generally mean the question is about a module, or the code used for a module.
As it is, the tag is more used to individuate questions that don't suite well Drupal Answers. If you see a question which is only using php, there are two cases: the OP doesn't know which tags should be used, or the question is too generic to be acceptable on Drupal Answers. If you see a question that uses different tags, and includes also php, it simply means the OP didn't read the excerpt that is shown when tags are selected; in this case, the tag should be simply removed.
The right thing to do with php would be asking for the tag to be burninated. If then the tag keeps to be used, we could ask it is black-listed.
Burninating a tag is done from Stack Exchange staff, which really burninates tag when there is a consensus about that. With the participation we have in this very site, I have not been successful on asking the burnination of api.