What is new in PHP 7.1 ?
Let me google that for you :) http://www.letmegooglethat.com/?q=What+is+new+in+PHP+7.1
One thing for sure (I recompiled PHP7 on my phone / Termux to run a webserver): native MySQL drivers. This is a major break from previous database methods. For what it's worth, I used MariaDB from Google Play and PHP + native MySQL was fine.
These questions are more appropriate to google yourself.