by Ken Task.
Just before Christmas Holidays in 2015 (I think) there was a 'gift' from Moodle.org folks ... 3.0.1 version of Moodle was verified to run under then 7.0.3 which was tagged by PHP folks as being development.
https://moodle.org/mod/forum/discuss.php?d=325088
Since then, because for the first time, Windows users have a *stable* 64bit version of PHP, under which they could run minimally 3.0.1+>.
Might want to check out the official PHP docs:
http://php.net/manual/en/migration70.php
where is does say:
"There are a few incompatibilities and new features that should be considered, and code should be tested before switching PHP versions in production environments."
http://php.net/manual/en/migration70.incompatible.php
Do have a 3.1.1 Moodle running on Google Apps engine CentOS 7 with PHP 7 and haven't seen issues ... other than those related to unoconv and Search.
Official Moodle: https://docs.moodle.org/dev/Moodle_and_PHP7
'spirit of sharing', Ken