<?php
session_start();
if($_GET['acao']=="logar"){
	$senha = $_POST['pwd'];
	if($senha=="trielo"){
		$_SESSION['userName'] = "UserNameXXX";
                $_SESSION['HTTP_USER_AGENT'] = md5($_SERVER['HTTP_USER_AGENT']);
   		header("Location: /home.php");
		exit;
	} else {
		echo "Senha incorreta!";
	}

}
?>

Iternas............................

if((!$_SESSION['userName'])||($_SESSION['HTTP_USER_AGENT'] != md5($_SERVER['HTTP_USER_AGENT']))){
  header("Location: /");
exit;
}