<?php
session_start();

//log out if ?logout=true
if($_GET["logout"]){
    $_SESSION["loggedIn"]=false;
    header('Location: '. $_SERVER['SCRIPT_NAME']);
}

//sha-256 hash of password
$password = '8f434346648f6b96df89dda901c5176b10a6d83961dd3c1ac88b59b2dc327aa4';

if(empty($_SESSION["loggedIn"])){
    if(isset($_POST["password"])){
        if(hash('sha256', $_POST['password'])==$password){
                $_SESSION["loggedIn"]=true;
        } else{
            echo "Incorrect password! <br>";
        }
    }
}

if($_SESSION["loggedIn"] == true){
print '
    You are logged in!<br>
    <a href="?logout=true">Logout</a>
    <!-- Main HTML page goes here-->
';
} else{
print ' <h3>Login</h3>
      <form action="#" method="post">
        Password: <input type="password" name="password"><br>
      <input type="submit" value="Login!" />
    </form>
';
}
?>

<?php
session_start();
if($_SESSION["loggedIn"]=false){
    header('Location: login.php');
} else {
    //stuff goes here
}