Initial commit

auth.php

+<?php
+
+define('PATH_TO_XML', __DIR__ . '/users.xml');
+
+function authenticateUser($login, $password) {
+	$users = simplexml_load_file(PATH_TO_XML);
+
+	foreach ($users as $user) {
+		if ($user->login == $login) {
+			return password_verify($password, $user->password);
+		}
+	}
+
+	return false;
+}
+
+function addUser($login, $password) {
+	$password = password_hash($password, PASSWORD_DEFAULT);
+
+	$users = simplexml_load_file(PATH_TO_XML);
+
+	$user = $users->addChild('user');
+	$user->addChild('login', $login);
+	$user->addChild('password', $password);
+
+	$users->asXML(PATH_TO_XML);
+}

index.php

+<?php
+session_start();
+
+if (!isset($_SESSION['logged_in']) || !$_SESSION['logged_in']) : ?>
+
+    <form action="login.php" method="post">
+        <input type="text" name="login" placeholder="User name"><br>
+        <input type="password" name="password" placeholder="Password"><br>
+        <a href="signup.php">Sign up</a>
+        <button>Log in</button>
+
+        <?php if (isset($_SESSION['login_failed']) && $_SESSION['login_failed']) : ?>
+            <br><span style="color: red">Incorrect login or password</span>
+        <?php $_SESSION['login_failed'] = false;
+        endif;
+        ?>
+    </form>
+
+<?php else : ?>
+
+    <form action="logout.php">
+        <button>Log out</button>
+    </form>
+
+    Secret content
+
+<?php endif; ?>

login.php

+<?php
+require 'auth.php';
+session_start();
+
+$login = $_POST['login'];
+$password = $_POST['password'];
+
+if (authenticateUser($login, $password)) {
+    $_SESSION['logged_in'] = true;
+} else {
+    $_SESSION['login_failed'] = true;
+}
+
+header("Location: index.php");
+die();

logout.php

+<?php
+session_start();
+
+$_SESSION['logged_in'] = false;
+
+header("Location: index.php");
+die();
\ No newline at end of file

signup.php

+<?php
+require 'auth.php';
+
+switch ($_SERVER['REQUEST_METHOD']) {
+    case 'GET': ?>
+
+        <form method="post">
+            <input type="text" name="login" placeholder="User name"><br>
+            <input type="password" name="password" placeholder="Password"><br>
+            <button>Sign up</button>
+        </form>
+
+        <?php
+        break;
+    case 'POST':
+        $login = $_POST['login'];
+        $password = $_POST['password'];
+
+        addUser($login, $password);
+
+        header("Location: index.php");
+        die();
+
+        break;
+}

users.xml

+<?xml version="1.0"?>
+<users>
+</users>