Undefined variable

[Al_katraz984]

<?php session_start();
?>
<form name="form" method="post" action="felice3.php">
<?
    $msg = '';
    if ($msg != "") {   
    print "[b]$msg[/b]

";
} else {
    print "[b]Inserire username e password[/b]

";
}    
?>
Username
<input name="username" type="text" id="username" size="15" maxlength="20">
Password
<input type="password" name="password" size="15" maxlength="20">
<input name="Submit" type="submit" class="submit" value="login">
</form>

E' buona norma definire sempre le variabili.. vuote, nulle, array che siano..