FreePBX works perfectly on PHP 7.1 (and even more so recently, as we’ve had some pull requests come through to fix up some warnings). We’d love to switch to that, but there are a couple of breaking back-end technical issues that are stopping us (that I am, actually, working on behind the scenes right now).
I wouldn’t be surprised if FreePBX 16 requires PHP 7, but that’s not an official statement, that’s just a feeling. The speed improvements alone are jaw-dropping.