the other part

<html>
	<head>
		<meta charset="utf-8">
		<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
		<script>
			$(document).ready(function() {
				$('#loginform').submit(function(e) {
					e.preventDefault();
					$.ajax({
						type: "POST",
						url: 'loginFinal.php',
						data: $(this).serialize(),
						success: function(data) {
							if (data === 'CORRECTO') {
								window.location = 'index.php';
							}
							else {
								alert('Invalid credentials');
							}
						}
					});
				});
			});

		</script>

	</head>
	<body>
		<form id="loginform" method="post">
		    Username: <input type="text" name="username" id="username" value="">

		    Password: <input type="password" name="password" id="password" value="">

		    <input type="submit" name="loginsub" id="loginsub" value="Login">
	    </form>
	</body>
</html>