Warning: Cannot modify header information – headers already sent by (output star

Hello, I am getting this problem

Warning: Cannot modify header information – headers already sent by (output started at D:\wamp\www\Sunil\wordpress2testing\wordpress-3.3.1\wordpress\wp-includes\post-template.php:924) in D:\wamp\www\Sunil\wordpress2testing\wordpress-3.3.1\wordpress\wp-includes\pluggable.php on line 669

Warning: Cannot modify header information – headers already sent by (output started at D:\wamp\www\Sunil\wordpress2testing\wordpress-3.3.1\wordpress\wp-includes\post-template.php:924) in D:\wamp\www\Sunil\wordpress2testing\wordpress-3.3.1\wordpress\wp-includes\pluggable.php on line 670

I have done all the things like white spaces <?php ?> after the tag. and i also search in the forum to sort out the problem. but i did not get any solution.please help me out and check my code below if there is any problem with my code.

It seems that i am getting the problem bcz of i am using the function wp_signon()….it works fine for only when username or password is wrong but when the username and password is correct then is throws error.

Thanks in advance.
avin.

<?php
if(is_user_logged_in())
{
//wp_redirect(home_url());
echo “hello”;
}
else
{
if($_POST)
{
$username = $_REQUEST[‘log’];
$password = $_REQUEST[‘pwd’];

$creds = array();
$creds[‘user_login’] = $username;
$creds[‘user_password’] = $password;
$creds[‘remember’] = true;
$user = wp_signon( $creds, false );
if ( is_wp_error($user) )
{
echo $user->get_error_message();
}

}
else
{
echo ‘<form name=”login” method=”post” id=”wp_login_form” action=””/>’;
echo ‘username’;
echo ‘<input type=”text” name=”log” id=”log” />’;
echo ‘password’;
echo ‘<input type=”text” name=”pwd” id=”pwd” />’;
echo ‘<input type=”submit” name=”submit” value=”log in”/>’;
echo ‘</form>’;
}

}
?>