Simple calendar class

[sonicfusion]

<?php

class calendar
{
	public $month;
	public $year;

public function __construct($month, $year)
{
	$this->month = $month;
	$this->year = $year;	
}

public function output()
{
	$date = mktime(0, 0, 0, $this->month, 1, $this->year);
	$week_num = date('W', $date);
	$first_day = date('N', $date) - 1;		
	$count_days = date('t', $date);
	$days = array(
		'Mon' => 1,
		'Tue' => 2,
		'Wed' => 3,
		'Thu' => 4,
		'Fri' => 5,
		'Sat' => 6,
		'Sun' => 7,
	);
	$str - '';
	$str .= '<h1>' . date('F Y', $date) . '</h1>';
	$str .= '<table>';
	$str .= '<tr>';
	$str .= '<th></th>';
	foreach($days as $day => $num)
	{
		$str .= '<th>' . $day . '</th>';
	}
	$str .= '</tr>';
	$day = 1;
	for($i = 0; $i < 6; $i++)
	{
		$str .= '<tr>';
		$str .= '<td><strong>' . $week_num . '</strong></td>';
		$week_num++;
		for($j = 0; $j <7; $j++)
		{
			if($first_day > 0)
			{
				$first_day--;
				$str .= '<td>';
				$str .= '</td>';
			}
			elseif($count_days > 0)
			{
				$str .= '<td>';
				$str .= $day++;
				$str .= '</td>'; 		
				$count_days--;		
			}
			else
			{
				$str .= '<td></td>';	
			}
		}
		$str .= '</tr>';
	}
	$str .= '</table>';
	return $str;
}
}
$calendar = new calendar(8, 2007);
echo $calendar->output();

?>

Quicky mashed this out...
How can I improve on this?
What would you do differently and why?

Thank you for your feedback.

[Weedpacket]

Unless this was going to get a lot more features in the future then it's no more than a plain function:

function calendar($month, $year)
    {
        $date = mktime(0, 0, 0, $month, 1, $year);
        $week_num = date('W', $date);
        $first_day = date('N', $date) - 1;        

        $count_days = date('t', $date);
        $days = array(
            'Mon' => 1,
            'Tue' => 2,
            'Wed' => 3,
            'Thu' => 4,
            'Fri' => 5,
            'Sat' => 6,
            'Sun' => 7,
        );
        $str - '';
        $str .= '<h1>' . date('F Y', $date) . '</h1>';
        $str .= '<table>';
        $str .= '<tr>';
        $str .= '<th></th>';
        foreach($days as $day => $num)
        {
            $str .= '<th>' . $day . '</th>';
        }
        $str .= '</tr>';
        $day = 1;
        for($i = 0; $i < 6; $i++)
        {
            $str .= '<tr>';
            $str .= '<td><strong>' . $week_num . '</strong></td>';
            $week_num++;
            for($j = 0; $j <7; $j++)
            {
                if($first_day > 0)
                {
                    $first_day--;
                    $str .= '<td>';
                    $str .= '</td>';
                }
                elseif($count_days > 0)
                {
                    $str .= '<td>';
                    $str .= $day++;
                    $str .= '</td>';         

                    $count_days--;        

                }
                else
                {
                    $str .= '<td></td>';    

                }
            }
            $str .= '</tr>';
        }
        $str .= '</table>';
        return $str;
    }

echo calendar(8,2007);