Version 5 and version 7 are completely different and separate extensions. As such, there is no direct upgrade path from version 5 to version 7. There is only clean installation of version 7 and then migration of the data from version 5 to version 7, using migration wizard
With that said, you can install both version 5 and version 7 extensions on the same site to test version 7. It should work with PHP5.6. But there is a strong possibility that something might not work correctly. To be honest, since PHP 5.6 has been retired more than three years ago, even before we started the development of version 6 (now version 7), you can imagine that we didn't care much about the compatibility with PHP 5.6
