There are:
Of the 9 php5 questions two are closed and none ask for anything specific to PHP5 (There's one comparison with earlier versions question, but it fits fine under php).
The distinction between PHP5 and earlier versions (namely 4) was important in the early days of PHP5 as it employed a completely rewritten engine which introduced quite a lot of compatibility issues. But it's not relevant any more, as PHP4 essentially reached it's end of life in 2008.
I think php5 doesn't make any sense on its own and probably never will, so we should merge it with php. I would have used the "suggest a synonym" thingy, but since php5 is version specific, it's up to the moderators.
I've scanned the popular language tags and I didn't find any other version specific tags lurking around, which is a good thing, we should probably avoid them as most questions that would fit them better than the language tag would probably be off topic.