Here is the raw way to do it
$startTimeStamp = strtotime("2011/07/01");
$endTimeStamp = strtotime("2011/07/17");
$timeDiff = abs($endTimeStamp - $startTimeStamp);
$numberDays = $timeDiff/86400; // 86400 seconds in one day
// and you might want to convert to integer
$numberDays = intval($numberDays);