Hi!
You cannot use php7 with FreePBX, for FreePBX 14 you must use PHP 5.6…
The main problem with making FreePBX work with PHP 7 is a dependency on a third party tool, Zen Guard, which is used for the commercial modules so until a solution is found, it won’t be actively supported.
That said, you can do this:
Good luck and have a nice day!
Nick