https://stackoverflow.com/a/1654008

number_of_days.php

<?php
define('ONE_WEEK', 604800); // 7 * 24 * 60 * 60

function number_of_days($days, $start, $end) {
    $w = array(date('w', $start), date('w', $end));
    $x = floor(($end-$start)/ONE_WEEK);
    $sum = 0;

    for ($day = 0;$day < 7;++$day) {
        if ($days & pow(2, $day)) {
            $sum += $x + ($w[0] > $w[1]?$w[0] <= $day || $day <= $w[1] : $w[0] <= $day && $day <= $w[1]);
        }
    }

    return $sum;
}

//$start = $end = time();

// 0x10 == pow(2, 4) == 1 << 4 // THURSDAY
// 0x20 == pow(2, 5) == 1 << 5 // FRIDAY
echo number_of_days(0x01, $start, $end); // SUNDAY
echo number_of_days(pow(2, 0), $start, $end); // SUNDAY
echo number_of_days(0x02, $start, $end); // MONDAY
echo number_of_days(pow(2, 1), $start, $end); // MONDAY
echo number_of_days(0x04, $start, $end); // TUESDAY
echo number_of_days(1 << 2, $start, $end); // TUESDAY
echo number_of_days(0x08, $start, $end); // WEDNESDAY
echo number_of_days(1 << 3, $start, $end); // WEDNESDAY
echo number_of_days(0x10, $start, $end); // THURSDAY
echo number_of_days(0x20, $start, $end); // FRIDAY
echo number_of_days(0x40, $start, $end); // SATURDAY
echo number_of_days(0x01 | 0x40, $start, $end); // WEEKENDS : SUNDAY | SATURDAY
echo number_of_days(0x3E, $start, $end); // WORKDAYS : MONDAY | TUESDAY | WEDNESDAY | THURSDAY | FRIDAY