I’m not aware of any other reports of a 14->15 upgrade failure like this, for the most part 14->15 is benign, there are zero OS changes in order to make it work. I just did a 14 install, fully upgraded the system and upgraded using the instructions on this wiki page with no major issues:
Upgrade to FreePBX 15 - FreePBX OpenSource Project - Documentation
I did run into this bug reported by Pitzkey for which there is an open ticket now, but I don’t know how you ended up with PHP 7. Your priority now will be to get a working system, which as stated above will involve a fresh install and restore the previously made backup. If you’re able to figure out what went wrong with the upgrade, please share it so we can either document or fix for others.