Sidebar login widget with error print, returns an error

hi to the community,

i’m building a widget to login from the sidebar and print errors if any,
so everything is working fine except that when i login it returns an error:
Warning: Cannot modify header information – headers already sent…..

the code of the widget:

<?php
// Custom Login/Meta Widget
function widget_artdev_meta() { ?>

<?php global $user_ID, $user_identity, $user_level ?>
<?php if ( $user_ID ) : ?>
<div class="widget">
	<h2>Control Panel</h2>
	<ul>
		<li>Identified as <strong><?php echo $user_identity ?></strong>.
		<ul>
			<?php if ( $user_level >= 10 ) : ?>
			<li><a href="<?php bloginfo('url') ?>/wp-admin">Dashboard</a></li>
			<?php endif // $user_level >= 10 ?>

			<?php if ( $user_level >= 1 ) : ?>
			<li><a href="<?php bloginfo('url') ?>/wp-admin/post-new.php">Write an article</a></li>
			<?php endif // $user_level >= 1 ?>

			<li><a href="<?php bloginfo('url') ?>/profile">Profile & personal options</a></li>
			<li><a href="<?php echo wp_logout_url( $_SERVER['REQUEST_URI'] ); ?>" title="Exit">Exit</a></li>
		</ul>
		</li>
	</ul>
</div>

<?php elseif ( get_option('users_can_register') ) : ?>

<?php
if ( 'POST' == $_SERVER['REQUEST_METHOD'] && !empty( $_POST['action'] ) && $_POST['action'] == 'log-in' ) :
	global $error;
	$login = wp_login( $_POST['log'], $_POST['pwd'] );
	$login = wp_signon( array( 'user_login' => $_POST['log'], 'user_password' => $_POST['pwd'], 'remember' => $_POST['rememberme'] ), false );
endif;
?>

<div class="widget">
	<h2>Identification</h2>

	<?php if ( $error ) : ?>
		<p class="error">
			<?php echo $error; ?>
		</p><!-- .error -->
	<?php endif; ?>

	<ul>
		<li>
		<form action="<?php the_permalink(); ?>" method="post">
			<label for="log">User ID
				<input type="text" name="log" id="log" value="<?php echo wp_specialchars(stripslashes($user_login), 1) ?>" size="22" class="input_text_login alignright" />
			</label>
			<label for="pwd">Password
				<input type="password" name="pwd" id="pwd" size="22" class="input_text_login alignright" />
			</label>
				<input type="submit" name="submit" value="Login" class="button" style="margin-right:35px; width:70px;" />
				<input name="rememberme" id="rememberme" type="checkbox" checked="checked" value="forever" /> Remember me
				<input type="hidden" name="action" value="log-in" />
				<input type="hidden" name="redirect_to" value="<?php echo $_SERVER['REQUEST_URI']; ?>"/>
		</form>
		</li>
		<li><a href="<?php bloginfo('url') ?>/wp-login.php?action=lostpassword">Recover password</a></li>
		<li><a href="<?php bloginfo('url') ?>/register">Register</a></li>
	</ul>
</div>

<?php endif // get_option('users_can_register') ?>
<?php }
if ( function_exists('register_sidebar_widget') )
register_sidebar_widget(__('→ Login/Meta Widget','artdev'), 'widget_artdev_meta');
?>

anyone out there can help?
i can’t understand where the problem is…!

thanks a lot!
Philip