PHP: Calculate number of weekdays between two dates.

date_diff_weekdays.php

<?php

    /**
     * Calculate number of weekdays between two dates.
     *
     * @param string $from  Start date, in format "YYYY-MM-DD"
     * @param string $to    End datem in format "YYYY-MM-DD"
     * @return int|null     Number of weekdays between $from and $to. NULL if either $from or $to is NULL.
     */
    static function date_diff_weekdays($from, $to) {
        if ($from === null || $to === null)
            return null;

        $date_from = new DateTime($from);
        $date_to = new DateTime($to);

        // calculate number of weekdays from start of week - start date
        $from_day = intval($date_from->format('w')); // 0 (for Sunday) through 6 (for Saturday)
        if ($from_day == 0)
            $from_day = 7;
        $from_wdays = $from_day > 5 ? 5 : $from_day;

        // calculate number of weekdays from start of week - end date
        $to_day = intval($date_to->format('w'));
        if ($to_day == 0)
            $to_day = 7;
        $to_wdays = $to_day > 5 ? 5 : $to_day;

        // calculate number of days between the two dates
        $interval = $date_from->diff($date_to);
        $days = intval($interval->format('%R%a')); // shows negative values too

        // calculate number of full weeks between the two dates
        $weeks_between = floor($days / 7);
        if ($to_day >= $from_day)
            $weeks_between -= 1;

        // complete calculation of number of working days between
        $diff_wd = 5 * ($weeks_between) + (5 - $from_wdays) + $to_wdays;

        return $diff_wd;
    }

This doesn't work properly.