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.