get_weeks_dates.php

<?php

function get_week_dates($month)
{
  $start = new DateTime($month.'-01');
  $end = new DateTime();
  $end->setDate($start->format('Y'), $start->format('m'), $start->format('t'));

  $interval = DateInterval::createFromDateString('1 day');
  $period = new DatePeriod($start, $interval, $end);

  $weeks = array();
  $week_key = 0;
  $first = true;
  foreach ($period as $dt) {    
    if ($first) {
      $weeks[$week_key]['start'] = $dt->format('Y-m-d');
      $first = false;
    }

    if ($dt->format('D') == 'Sun' || $dt->format('d') == $end->format('d')) {
      $weeks[$week_key]['end'] = $dt->format('Y-m-d');
      $week_key++;
      $first = true;
    }
  }
  return $weeks;
}

$weeks = get_week_dates('2021-01');

var_dump($weeks);