calc days of week in month

dow.php

<?php

function calcDays($daysToCalc, string $firstDay, string $lastDay)
  {
    $start = new \DateTime($firstDay);
    $oneDay = new \DateInterval('P1D');

    $end = (new \DateTime($lastDay))->sub($oneDay);

    $day = $start;
    $count = 0;

    while ($day <= $end) {
      if (in_array((int)$day->format('w'), $daysToCalc)) {
        $count++;
      }
      $day = $day->add($oneDay);
    }
    
    echo $count . PHP_EOL;
  }