<?php

session_start();

// DECLARE VARIABLES //

$username = 'username';
$password = 'password';

$self = $_SERVER['REQUEST_URI'];

// USER LOGOUT //

if(isset($_GET['logout']))
{
	unset($_SESSION['login']);
}

// USER IS LOGGED IN //

if (isset($_SESSION['login'])) {

?>

		echo '<p>Hello World</p>';
		<a href="?logout=true">Logout?</a>

	<?php
}

// FORM HAS BEEN SUBMITTED //

else if (isset($_POST['submit'])) {

	if ($_POST['username'] == $username && $_POST['password'] == $password){

		//IF USERNAME AND PASSWORD ARE CORRECT //
		header("Location: $_SERVER[PHP_SELF]");
		//echo '<p>Hello World'<p>;

	} else {

		// DISPLAY FORM WITH ERROR //
		display_login_form();
		echo '<p>Username or password is invalid</p>';

	}
}

// SHOW THE LOG-IN FORM //

else {

	display_login_form();

}


function display_login_form(){ ?>

	<form action="<?php echo $self; ?>" method='post'>
	<label for="username">username</label>
	<input type="text" name="username" id="username">
	<label for="password">password</label>
	<input type="password" name="password" id="password">
	<input type="submit" name="submit" value="submit">
	</form>

<?php } ?>