Nulla, continua a restituirmi lo stesso errore su queste due righe

codice:
		$user=trim($_POST["username"]);
		$pwd=trim($_POST["password"]);
codice:
<?php
    session_start();
    unset($_SESSION["collegato"]);
	if (isset($_POST))
	{
		$user=trim($_POST["username"]);
		$pwd=trim($_POST["password"]);
		$mex="";
		if($user!=""&&$pwd!="")
		{
			if ($user=="utente"&&$pwd=="password")
			{
				$_SESSION["collegato"]="si";
				header("location: index.php");
				die("...Attendere, prego...");
			}
			else $mex="<br />Errore: Username o Password errate!<br /> <br />";
		}
	}
?>
<html>
<head>
    <title>Area riservata</title>
</head>
<body>
<strong>Login</strong>
<br />
<?php if($mex!="") echo $mex;?>
<form method="post" action="login.php" enctype="multipart/form-data">
Username: <input type="Text" name="username" maxlength="20" size="10" />
<br />
Password: <input type="Password" name="password" maxlength="20" size="10" />
<br />
<input type="Submit" value="Entra" />
</form>
</body>
</html>