index.php

<?php
// We need to use sessions, so you should always start sessions using the below code.
session_start();
// If the user is not logged in redirect to the login page...
if (isset($_SESSION['loggedin'])) {
	header('Location: upload.php');
	exit();
}
?>
<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title>Login</title>
		<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">
		<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.3.1/dist/css/bootstrap.min.css">
		<link rel="stylesheet" href="css/style.css">
	</head>
	<body>
		<div class="login">
			<h1>Login</h1>
			<form action="authenticate.php" method="post">
				<label for="username">
					<i class="fa fa-user"></i>
				</label>
				<input type="text" name="username" placeholder="Username" id="username" required>
				<label for="password">
					<i class="fa fa-lock"></i>
				</label>
				<input type="password" name="password" placeholder="Password" id="password" required>
				<input type="submit" class="btn btn-dark" value="Login">
			</form>
		</div>
	</body>
</html>