<?php

// your code goes here

$startTimeStamp = strtotime("2016/01/06");
$endTimeStamp = strtotime("2016/01/08");

$timeDiff = abs($endTimeStamp - $startTimeStamp);

$numberDays = $timeDiff/86400;  // 86400 seconds in one day

// and you might want to convert to integer
$numberDays = intval($numberDays);

echo $numberDays;