function selected($o,$selected)
    {        
        foreach ($o as $value => $seen )
        {
            if($selected == $value )
            { 
                echo "<option value=\"$value\" selected=\"selected\">$seen</option>";
            }else
            {
                echo "<option value=\"$value\">$seen</option>";
            }            
        }     
    }

if ($_POST['hesapla']) {
$posttan_gelen_ay = $_POST['ay'];

}else {

$posttan_gelen_ay = '1';
}

$aylar= array('1'=>'1',
                '2'=>'2',
                '3'=>'3',
                '4'=>'4',
                '5'=>'5',
                '6'=>'6',
                '7'=>'7',
                '8'=>'8',
                '9'=>'9',
                '10'=>'10',
                '11'=>'11',
                '12'=>'12');
                
                
echo '<select name="tarih" >';


selected($aylar,$posttan_gelen_ay);

echo '</select>';