I don't think php-library is a good tag. It is ambiguous, provides no information that doesn't have to be clarified in the question and can be misleading. Are they talking about the SPL, their own library, a third-party library or an extension? Either way, it is something that needs to be clarified with another tag or content in the question.
There are currently only 16~ questions with this tag. I am willing to go through and manually retag these to php, I am simply not sure if it would be appropriate to retag that many questions without community feedback.