Login page coding

<?php
session_start();
session_regenerate_id(true);
$username = $_POST['username'];
$password = $_POST['password'];
$login = $_SERVER['HTTP_REFERER'];
$_SESSION["username"] = "$username";

if( ( !$username) or (! $password ))
{header ("Location:$login"); exit(); }

$conn=@mysql_connect( "localhost", "root", "")
or die ("cannot connect");

$rs = @mysql_select_db("dbusers", $conn)
or die("database problem");

$sql = "select * from dropbox where uname=\"$username\" and pass = \"$password\"";

$rs = mysql_query($sql, $conn)
or die ("could not execute query");

$rows = mysql_numrows( $rs);

if($rows !=0)
{
echo("<h2>$username okay - !welcome!</h2>");

}
else
{
header("Location:$login"); exit();
}


?>
<html>

<body>
Session usr =
<?php
echo(session_id());
?>
<br><br>

<a href="home.php?<?php echo(SID); ?>">Go to next page</a>

<a href="home.php">Home</a>

</body>

</html>