Errore Form Login: "Cannot Modify Header"

[Dexter1983]

grazie, ma ancora non funziona:

<div>
<ul>
<form method="POST">
<table>
<tr><td>Username : </td><td><input type="text" name="user" /></td></tr>
<tr><td>Password : </td><td><input type="password" name="password" /></td></tr>
<tr><td><input type="submit" name="action" value="Login" /></td></tr>
</table>
</form>[/list]
<?php
$chkusr="nome";
$chkpas="pass";
if (isset($_POST['action']) and
    isset($_POST['user']) and
    isset($_POST['password']) and
    $_POST['user']==$chkusr and
    $_POST['password']==$chkpas)
header ("Location:/index.php?mnuItm=modify"); //
?>
</div>

l'errore dice che ho gia inviato.. ma che cosa?