Enable SNMP php 7 Ubuntu

Before you enable SNMP in PHP 7, you first need to make sure that you have the proper SNMP packages installed on the Ubuntu System.

Install the SNMP packages

sudo apt-get install snmpd snmp libsnmp-dev

Before you install the PHP package, make sure you download and install the following package:

apt-get install snmp-mibs-downloader

This package will help translate OIDs into human readable strings

Now you can install the php extension.

apt-get install php-snmp

Then restart the followings:

service snmpd restart
service apache2 restart
or
Service nginx restart

Reference
Install and Manage MIBS file
https://packages.ubuntu.com/xenial/net/snmp-mibs-downloader