Calculating the number of Saturdays and Sundays

<?php
date_default_timezone_set("Europe/Lisbon");
$d1 = new DateTime("2009-06-01"); /* inclusive */
$d2 = new DateTime("2009-07-01"); /* exclusive */

$interval = $d2->diff($d1);
$number_of_days = $interval->format("%d");

$number_of_weekends = $number_of_days / 7;
$remainder = $number_of_days % 7;

if ($remainder >=2 && $d1->format("D") == "Sat")
    $number_of_weekends++;
elseif ($d1->format("w") + $remainder >= 8)
    $number_of_weekends++;

I may have missed by one in the last condition, be sure to check it with different starting dates. (Feel free to edit this answer if you spot an error).