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auth.php

<?php

class Auth {
  /**
   * Our unique session key
   */
  protected $key = '__MY_APP_AUTH__';

  /**
   * PDO Instance
   */
  protected $db;

  /**
   * Our user's data
   */
  protected $data;

  public function __construct() {
    $this->db = new PDO('mysql:host=localhost;port=3600', 'root', 'root')
    $this->user()
  }

  /**
   * Check if a user is authenticated
   */
  public function check() {
    return null != $this->data;
  }

  /**
   * Opposite of `check`. Check if a user is not authenticated.
   */
  public function guest() {
    return null == $this->data;
  }

  /**
   * Get the user data
   */
  public function user() {
    if ($this->data) {
      return $this->data;
    }

    $prepared = $this->db->query("SELECT * FROM users WHERE username = :id LIMIT 1")
    return $this->data = $prepared->query(['id' => $_SESSION[$this->key]]);
  }

  /**
   * Login the user with the provided credentials
   */
  public static function login($credentials) {
    $prepared = $this->db->query("SELECT * FROM users WHERE username = :username LIMIT 1")
    $user = $prepared->query(['username' => $credentials['username']])

    if (null == $user) {
      return false;
    }

    if ($credentials['password'] !== $user->password) {
      return false;
    }

    $_SESSION[$this->key] = $this->data->id
    $this->data = $user;

    return true;
  }

  /**
   * Logout the authenticated user
   */
  public static function logout() {
    delete $_SESSION[$this->key];
    $this->data = null;
  }
}

$auth = new Auth();
$auth->guest()
$auth->check()
$auth->login(['username' => 'admin', 'password' => '12369']);
$auth->user()
$auth->logout()