1- how to make a plugin support php7 2- where to learn php7 3- what is pocketmine api diffrents in php7
to make the plugin php7 just change api number to 2.0.0 and you'll only have to make minimal to none changes in code
To extend on what Michael said above, as far as I know, you need to change the NBT names since PHP7 disallows you to use String/Enum/Double/Etc as a class name.