a richiamo dal form se é corretto:

Codice PHP:
<html>
<head>
<title>Area Riservata</title>
</head>

<body>
<form name="login" method="post" action="<?php echo $_SERVER['PHP_SELF'];?>" [COLOR=red]onSubmit="return checkLogin()[/COLOR]">
<table width="400" border="0" cellpadding="0" cellspacing="0">
<tr>
    <td width="200">Username:</td>
    <td width="200"><input name="usr" type="text" id="usr" /></td>
</tr>
<tr>    
    <td>Password:</td>
    <td><input name="pwd" type="password" id="pwd" /></td>
</tr>
<tr>
    <td><?php echo $msg;?></td>
    <td><input type="Submit" value="Submit" name="Submit"></td>
</tr>
</table>
</form>
</body>
</html>
la richiamo cosi onsubmit="return checkLogin();"

Grazie Michel