I agree with the others, just learn PHP (period). That is, PHP, the latest.
But I will also say, many cheap hosts are not on PHP 7 yet, which means if you are going to build stuff and stick it on a cheap host, you may want to make yourself aware of some of the primary changes between 5.6 and 7.
It's not a lot, some of it is just syntax sugar and shortcuts. So go ahead and study PHP, meaning PHP "the latest one". But then at some point take a step back and read a two or three page document that summarizes the new features of 7 so you know what they are.