How to create a calendar in php

Calendar.php

<?php
class Calendar
{
    private $month;
    private $date;
    private $events = [];

    public function __construct($date)
    {
        $this->date = new \DateTime($date);
        $this->month = $this->getMonth();

        $this->setFirstDay();
    }

    public function next()
    {
        return $this->date->modify("+ 1 day");
    }

    public function getDay()
    {
        return $this->date->format("j");
    }

    public function isCurrentMonth()
    {
        return $this->date->format("n") == $this->month;
    }

    public function addEvent($day, $data)
    {
        $this->events[$day][] = $data;
    }

    public function getEvents()
    {
        //only shows events of the current month
        if (!isset($this->events[$this->getDay()]) || !$this->isCurrentMonth()) {
            return [];
        }

        return $this->events[$this->getDay()];
    }

    private function getMonth()
    {
        return $this->date->format("n");
    }

    /*
     * sets the initial date as the first Monday of the calendar(usually not the first day of the current month but the last days of the previous month)
     */
    private function setFirstDay()
    {
        $this->date->modify('first day of this month');
        $firstWeekDay = $this->getFirstWeekDay();

        $this->date->modify("-{$firstWeekDay} days");
    }

    /*
     * weekday (0 for Sunday)
     */
    private function getFirstWeekDay()
    {
        return $this->date->format("w");
    }
}


$calendar = new Calendar($_GET['date']);
$event = [
    'name' => "Take a nap",
    'date' => '16/10/2020',
    'time' => '14:00',
];

$calendar->addEvent(1, $event);

$event = [
    'name' => "Wakeup",
    'date' => '16/10/2020',
    'time' => '15:00',
];
$calendar->addEvent(1, $event);

?>
<table border="1">
    <thead>
    <tr>
        <th>D</th>
        <th>S</th>
        <th>T</th>
        <th>Q</th>
        <th>Q</th>
        <th>S</th>
        <th>S</th>
    </tr>
    </thead>

    <?php for ($week = 0; $week < 6; $week++) { ?>
        <tr>
            <?php for ($day = 0; $day < 7; $day++) { ?>
                <?php $date = $calendar->next() ?>
                <td <?php if (!$calendar->isCurrentMonth()) { ?> style="background: gray" <?php } ?>>
                    <?= $calendar->getDay() ?>
                    <?php foreach ($calendar->getEvents() as $event) { ?>
                        <div><?= $event['name'] ?></div>
                    <?php } ?>
                </td>
            <?php } ?>
        </tr>
    <?php } ?>
</table>
<?php
?>