codigo php validacion de usuarios

para validar usuarios en php les entrego el siguiente codigo
// *** Validate request to login to this site.
session_start();

$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($accesscheck)) {
$GLOBALS['PrevUrl'] = $accesscheck;
session_register('PrevUrl');
}

if (isset($_POST['nick'])) {
$loginUsername=$_POST['nick'];
$password=$_POST['password'];
$MM_fldUserAuthorization = "";
$MM_redirectLoginSuccess = "SENA.php";
$MM_redirectLoginFailed = "sorry.htm";
$MM_redirecttoReferrer = true;
mysql_select_db($database_tres, $tres);

$LoginRS__query=sprintf("SELECT Nick, Password FROM registro WHERE Nick='%s' AND Password='%s'",
get_magic_quotes_gpc() ? $loginUsername : addslashes($loginUsername), get_magic_quotes_gpc() ? $password : addslashes($password));

$LoginRS = mysql_query($LoginRS__query, $tres) or die(mysql_error());
$loginFoundUser = mysql_num_rows($LoginRS);
if ($loginFoundUser) {
$loginStrGroup = "";

//declare two session variables and assign them
$GLOBALS['MM_Username'] = $loginUsername;
$GLOBALS['MM_UserGroup'] = $loginStrGroup;

//register the session variables
session_register("MM_Username");
session_register("MM_UserGroup");

if (isset($_SESSION['PrevUrl']) && true) {
$MM_redirectLoginSuccess = $_SESSION['PrevUrl'];
}
header("Location: " . $MM_redirectLoginSuccess );
}
else {
header("Location: ". $MM_redirectLoginFailed );
}
}