weeknumber not rigth

<?php
if (isset($_GET['month']) && isset($_GET['year']))
	{
	
	$this_month = cleanQuery($_GET['month']);
	$this_year = cleanQuery($_GET['year']);

    	if ($this_month == 13) 
		{
       		$this_month = 1;
        	$this_year = $this_year + 1;
    		}
    	elseif ($this_month == 0) 
		{
        	$this_month = 12;
        	$this_year = $this_year - 1;
        	}
    	else 
		{
             	$this_month = $this_month;
        	}
     	}

else 	
	{
	$this_month = date("n");
	$this_year = date("Y");
	}

?>
<?php 

$days_of_the_month = date("t", mktime(0, 0, 0, $this_month, 1, $this_year));
$first_weeknumber_of_the_month = strftime("%W", mktime(0,0,0,$this_month,1,$this_year));
$last_weeknumber_of_the_month = strftime("%W", mktime(0,0,0,$this_month,$days_of_the_month,$this_year));
$weeknumber = $first_weeknumber_of_the_month;

echo "<ul id=\"weeknumbers\">";
echo "<li>W</li>";
while ($weeknumber <= $last_weeknumber_of_the_month)
		{
		echo "<li>".$weeknumber."</li>";
		$weeknumber++;
		}

echo "</ul>";

?>