The porter should be able to work with PHP 7.x since October 2018. But the version you can download here doesn't seem to be the most recent version... To make your first steps quickly, get the files from GitHub and build the importer script by yourself:
The porter should run fine with PHP 7.x. It looks like we haven’t pushed an update here in some time. In the meantime go get it from the github repo. I’ll update here when after I upload the latest version to the adding directory.