[PHP] Login and Security help

[davidflatman]

I have made a posting script (a kind of shout box system)
I need to have a way to make sure that whatever script is being processed is coming from the same server (i know there is a way to do this) but how?
and also i need a login system.. if i was to implement a system such as this...

session_start();
if (isset($_POST['user']) && isset($_POST['pass']))
{
	if ($_POST['user'] === 'user' && $_POST['pass'] === 'pass')
	{
		$_SESSION['aloud '] = true;
			header('Location: protectedish.php');
			exit;
	}
		else 
		{
		$wrong = 'Wrong User/Password';
		}
}

and then this on each "protected" page...

session_start();
if (!isset($_SESSION['aloud']) or $_SESSION['aloud'] !== true)
	{
		header('Location: login.php');
		exit;
	}

I am assuming this is not very secure, and it is also inconvenient, for instance if i need to use another "header('location:')" somewhere else it won't work.

bearing in mind that i only need one user, and i would like a system where the user can easy change the password, what would be a good system? and what are other methods to keep my scripts secure?

sorry for all those questions