<?php
 
declare(strict_types=1);
 
if (function_exists('date_default_timezone_set')) {
  // Set the default timezone to US/Eastern
  date_default_timezone_set('US/Eastern');
}
 
/**
 * Will return a timestamp of the last day in a month for a specified year
 *
 * @param int $month
 * @param int $year
 *
 * @return false|int
 */
function last_day(int $month, int $year):int {
  // Use mktime to create a timestamp one month into the future, but one
  // day less. Also make the time for almost midnight, so it can be
  // used as an 'end of month' boundary
  return mktime(23, 59, 59, $month + 1, 0, $year);
}
 
/**
 * This function will generate and return an HTML Calendar
 *
 * @param int $month
 * @param int $year
 * @param int $weekdaytostart
 *
 * @return string[]
 */
function get_html_calendar(int $month, int $year, int $weekdaytostart = 0):array {
  /** @var string[] $calendar */
  $calendar = [];
 
  // There are things we need to know about this month such as the last day:
  $last = idate('d', last_day($month, $year));
 
  // We also need to know what day of the week the first day is, and let's
  //  let the system tell us what the name of the Month is:
  $firstdaystamp = mktime(0, 0, 0, $month, 1, $year);
  $firstwday = idate('w', $firstdaystamp);
  $name = date('F', $firstdaystamp);
 
  // To easily enable our 'any day of the week start', we need to make an
  //  array of weekday numbers, in the actual printing order we are using
  $weekorder = [];
 
  for ($wo = $weekdaytostart; $wo < $weekdaytostart + 7; $wo++) {
    $weekorder[] = $wo % 7;
  }
 
  // Now, begin our HTML table
  $calendar[] = sprintf('<table><tr><th colspan="7">%s %s</th></tr>', $name, $year);
 
  // Now before we really start, print a day row:
  // Use the system to tell us the days of the week:
  $calendar[] = '<tr>';
 
  // Loop over a full week, based upon day 1
  foreach ($weekorder as $w) {
    $dayname = date('D', mktime(0, 0, 0, $month, 1 - $firstwday + $w, $year));
    $calendar[] = sprintf('<th>%s</th>', $dayname);
  }
 
  $calendar[] = '</tr>';
 
  // Now we need to start some counters, and do some looping:
  $onday = 0;
  $started = FALSE;
 
  // While we haven't surpassed the last day of the month, loop:
  while ($onday <= $last) {
    // Begin our next row of the table
    $calendar[] = '<tr>';
 
    // Now loop 0 through 6, for the days of the week, but in the order
    //  we are actually going, use mod to make this work
    foreach ($weekorder as $d) {
      // If we haven't started yet:
      // Does today equal the first weekday we should start on?
      if (!($started) && $d === $firstwday) {
        // Set that we have started, and increment the counter
        $started = TRUE;
        $onday++;
      }
 
      // Now if the day is zero or greater than the last day make a
      //  blank table cell.
      if (($onday === 0) || ($onday > $last)) {
        $calendar[] = '<td>&nbsp;</td>';
      }
      else {
        // Otherwise, echo out a day & Increment the counter
        $calendar[] = sprintf('<td>%s</td>', $onday);
        $onday++;
      }
    }
 
    // End this table row:
    $calendar[] = "</tr>";
  }
 
  // Now end the table:
  $calendar[] = '</table>';
 
  return $calendar;
}
 
// ========================================================
// = Demo showing calendar for all months of current year =
// ========================================================
 
// Output some formatting directives:
echo '<style>table, td, th { border: 1px solid #000; }</style>';
 
// Create an entire year calendar for 2006 with Monday as the first day:
$current_year = (int) date('Y');
foreach (range(1, 12) as $month) {
  echo implode('', get_html_calendar($month, $current_year, 1));
}