Calendar.php

<?php

/**
 * Class Calendar
 *
 * @author Luciano Charles de Souza
 * E-mail: souzacomprog@gmail.com
 * Github: https://github.com/LucianoCharlesdeSouza
 * YouTube: https://www.youtube.com/channel/UC2bpyhuQp3hWLb8rwb269ew?view_as=subscriber
 */

class Calendar
{

  public function month($month)
  {
    $months = array(
      '01' => "Janeiro", '02' => "Fevereiro", '03' => "Março", '04' => "Abril", '05' => "Maio", '06' => "Junho",
      '07' => "Julho", '08' => "Agosto", '09' => "Setembro", '10' => "Outubro", '11' => "Novembro", '12' => "Dezembro"
    );

    if ($month >= 01 && $month <= 12) {
      return $months[$month];
    }

    return "Mês deconhecido";
  }

  public function days($month)
  {
    $days = array(
      '01' => 31, '02' => 28, '03' => 31, '04' => 30, '05' => 31, '06' => 30,
      '07' => 31, '08' => 31, '09' => 30, '10' => 31, '11' => 30, '12' => 31
    );

    if (((date('Y') % 4) == 0 and (date('Y') % 100) != 0) or (date('Y') % 400) == 0) {
      $days['02'] = 29;
    }

    return $days[$month];
  }

  public function week()
  {
    $days = "DSTQQSS";
    $week = [];

    for ($i = 0; $i < 7; $i++) {
      $week[] =  "<td>" . $days{$i} . "</td>";
    }

    return implode('', $week);
  }

  public function amountOfWeeks($month)
  {

    return jddayofweek(cal_to_jd(CAL_GREGORIAN, $month, "01", date('Y')), 0);
  }

  public function render($month)
  {
    $days = $this->days($month);
    $nameMonth = $this->month($month);
    $dayWeek = $this->week();
    $amountOfWeeks = $this->amountOfWeeks($month);


    return $this->makeTable($nameMonth, $days, $dayWeek, $amountOfWeeks);
  }

  private function makeTable($nameMonth, $days, $dayWeek, $amountOfWeeks)
  {
    $dayCurrent = 0;
    $increment = 0;

    $tableOpen =  "<table style='text-align:center;'><tr><td style='font-size:1.4em;'>{$nameMonth}</td></tr><tr><td></td>{$dayWeek}</tr>";
    $tableBody = '<tr>';
    $tableClose = '</table>';

    $tableBody .= $this->makeLineDays($increment, $dayCurrent, $days, $tableBody, $amountOfWeeks);

    return  $tableOpen . $tableBody . $tableClose;
  }

  private function makeLineDays($increment, $dayCurrent, $days, $tableBody, $amountOfWeeks)
  {
    for ($line = 0; $line < 6; $line++) {

      $increment++;

      if ($dayCurrent + 1 <= $days) {

        $tableBody .= "<td style='background-color:#ccc; padding:5px;'>Semana {$increment}</td>";

        for ($column = 0; $column < 7; $column++) {

          if ($dayCurrent + 1 <= $days) {

            $tableBody .= "<td style='width:30; height:30; background-color:#f3f3f3; padding: 5px'>";

            if ($column < $amountOfWeeks && $line == 0) {

              $tableBody .= ' ';
            } else {

              $tableBody .= ++$dayCurrent;
              $tableBody .= "</td>";
            }
          } else {
            break;
          }
        }
      }

      $tableBody .= "</tr>";
    }

    return $tableBody;
  }
}

date_default_timezone_set('America/Sao_Paulo');

$calendar = new Calendar;

echo $calendar->render('11');