login.php

<?php

session_start();

if (isset($_SESSION['login'])){
	session_destroy();
	header("Location: login.php");
}

if (isset($_POST['submit'])){
	$_SESSION['login'] = true;
	header("Location: masuk.php");
	exit;
}
?>
<form method="post">
	<button type="submit" name="submit">Login</button>
</form>

masuk.php

<?php

session_start();

if (isset($_SESSION['login'])){
	echo 'udah login';
} else {
	header("Location: login.php");
	exit;
}