Hello Ginji,
like d00p wrote, php version 7 or higher is required to run Froxlor 10. You are running Debian Jessie, and although that is long-term supported until June 2020 (see https://www.debian.org/releases/jessie/ for more info on this), I think you won't get php7 or greater packages for it (please see https://packages.debian.org/search?keywords=php7&searchon=names&suite=all§ion=all for more on this).
So my suggestion would be to also upgrade Debian itself to the latest stable version (currently 10, codename "Buster"). I did, and Froxlor 0.10.1 runs very nicely on it.
Hope that helps,
and cheers,
Wolfgang