periodIterator.php

<?php

class PeriodIterator implements \Iterator
{
    private $current;
    private $period = [];

    public function __construct(\DatePeriod $period) {
        $this->period = $period;
        $this->current = $this->period->getStartDate();
        if(!$period->include_start_date){
            $this->next();
        }

        $this->endDate = $this->period->getEndDate();
    }

    public function rewind() {
        $this->current->subtract($this->period->getDateInterval());
    }

    public function current() {
        return clone $this->current;
    }

    public function key() {
        return $this->current->diff($this->period->getStartDate());
    }

    public function next() {
        $this->current->add($this->period->getDateInterval());
    }

    public function valid() {
        return $this->current < $this->endDate;
    }

    public function extend()
    {
        $this->endDate->add($this->period->getDateInterval());
    }
}

$period = new \DatePeriod($startDateTime, new \DateInterval('P1D'), $endDateTime);

$periodIterator = new PeriodIterator($period);
$dueDate = clone $startDateTime;
while($periodIterator->valid()){
    $dueDate = $periodIterator->current();
    if(self::isSaturday($dueDate) || self::isSunday($dueDate)){
        $periodIterator->extend();
    }
    $periodIterator->next();
}

$dueDate->setTime(18, 0, 0);