Even though no questions exist anymore in the system with the php6 tag, when using the synonym link, it says it already exists.
Since there are no questions tagged php6, it will automagically be removed from the system eventually.
I think you are confused with the message you got. The tag doesn't currently exist, and that is the problem. Read the error message more carefully:
I'm not sure if there is a real benefit to having a PHP6 tag synonym since PHP6 doesn't actually exist yet, which is why the tag was killed off in the first place. Chances are if a person is coming here to ask a question about PHP6, it probably doesn't belong here.