Well,
I went through this to install php7 and update the whole thing.
First follow this post to install php7: https://www.digitalocean.com/community/tutorials/how-to-upgrade-to-php-7-on-ubuntu-14-04
Then (after searching at mailinabox/setup) installed all this modules:
php7.0-cli php7.0-sqlite php7.0-mcrypt php7.0-intl php7.0-json php7.0-common php-auth php-net-smtp php-net-socket php-net-sieve php-mail-mime php-crypt-gpg php7.0-gd php7.0-pspell tinymce libjs-jquery libjs-jquery-mousewheel libmagic1 php7.0-mbstring php7.0-curl
Then repaired z-push problem following this sha1sum: WARNING: 1 computed checksum did NOT match
And finally launching update (it doesn’t matter if already done) with curl -s https://mailinabox.email/setup.sh | sudo bash
All the steps breaks something, one way or another, and the update finally sets everything in its place. I recommend to take a snapshot first. This leaves the system with php7 working for MiAB.
Good luck,