<?php
$year               = 2018;
$month              = 12;
$first_day_of_month = "$year-$month-01";
$ts                 = strtotime($first_day_of_month);
$day_of_week_number = date("w", $ts); // 0-6
$last_day_of_month  = date('t', $ts); // e.g. 31

$days = range(1, $last_day_of_month);
$grid = array_fill(0, 42, null);
array_splice($grid, $day_of_week_number, count($days), $days);
$grid = array_chunk($grid, 7);

echo '<table>';
echo '<thead><tr><th>';
echo implode('</th><th>', ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']);
echo '</th></tr></thead><tbody>';
foreach($grid as $row) {
    echo '<tr><td>' . implode('</td><td>', $row) . '</td></tr>';
}
echo '</tbody></table>';