- abs: documentation ( source)
- strtotime: documentation ( source)
<?php
$startTimeStamp = strtotime("2011/07/01");
$endTimeStamp = strtotime("2011/07/17");
$timeDiff = abs($endTimeStamp - $startTimeStamp);
$numberDays = $timeDiff/86400;
// and you might want to convert to integer
$numberDays = intval($numberDays);