function filter_var missing

https://www.howtoforge.com/tutorial/perfect-server-ubuntu-16.04-with-apache-php-myqsl-pureftpd-bind-postfix-doveot-and-ispconfig/

Ubuntu 16.04 php7

When trying to change or create a new mailbox, the following error occurs:

function filter_var missing

/usr/local/ispconfig/interface/lib/classes/tform_base.inc.php

line 1015

if(function_exists('filter_var') && $field_value < 2147483647) {

This checks that causes the error: "$field_value < 2147483647"