alright - running latest Piwik on PHP 7.0.7 since yesterday - and no problems so far!
Just had to manually compile PECL GEOIP from latest SVN Trunk (Release wonβt compile with PHP 7) --> PHP: Index of /pecl/geoip/trunk
I almost caved upgrading to PHP 7.0.7 from PHP 5.4 in preparation for Piwik 3.0 as there were 2 errors I could not get around. However, we were finally successful upgrading PHP with both PHP7.0.13 and PHP7.0.14. After the upgrade, we needed to boost our memory to 2G as we blew past the default allowed memory size. Hope this helps.